The Astronomical Messenger

THE COSMIAN STARS.³

THE COSMIAN STARS.³

IN the present small treatise I set forth some matters of great interest for all observers of natural phenomena to look at and consider. They are of great interest, I think, first, from their intrinsic excellence; secondly, from their absolute novelty; and lastly, also on account of the instrument by the aid of which they have been presented to my apprehension.

In this short work, I present some matters of great interest for all observers of nature to examine and consider. I think these matters are of great interest for three reasons. First, they are remarkable in themselves. Second, they are entirely new. Finally, they are also interesting because of the instrument that allowed me to observe them.

The number of the Fixed Stars which observers have been able to see without artificial powers of sight up to this day can be counted. It is therefore decidedly a great feat to add to their number, and to set distinctly before the eyes other stars in myriads, which have never been seen before, and which surpass the old, previously known, stars in number more than ten times.

The fixed stars observers have seen with the naked eye up to now can be counted. So adding to their number is truly a great achievement. So is bringing tens of thousands of previously unseen stars clearly into view. These new stars outnumber the previously known stars by more than ten to one.

Again, it is a most beautiful and delightful sight to behold the body of the Moon, which is distant from us nearly sixty semi-diameters⁴ of the Earth, as near as if it was at a distance of only two of the same measures; so that the diameter of this same Moon appears about thirty times larger, its surface about nine hundred times, and its solid mass nearly 27,000 times larger than when it is viewed only with the naked eye; and consequently any one may know with the certainty that is due to the use of our senses, that the Moon certainly does not possess a smooth and polished surface, but one rough and uneven, and, just like the face of the Earth itself, is everywhere full of vast protuberances, deep chasms, and sinuosities.

Again, it is a beautiful and delightful sight to see the Moon so close. The Moon is nearly sixty semi-diameters⁴ of the Earth away, yet appears as if only two such measures away. As a result, the Moon’s diameter appears about thirty times larger than when seen with the naked eye alone. Its surface appears about nine hundred times larger, and its solid mass nearly 27,000 times larger. Consequently, anyone can know, with the certainty our senses provide, that the Moon’s surface is certainly not smooth and polished. Instead, the surface is rough and uneven, just like the Earth’s surface, and everywhere full of vast bulges, deep chasms, and winding features.

Then to have got rid of disputes about the Galaxy or Milky Way, and to have made its nature clear to the very senses, not to say to the understanding, seems by no means a matter which ought to be considered of slight importance. In addition to this, to point out, as with one’s finger, the nature of those stars which every one of the astronomers up to this time has called nebulous, and to demonstrate that it is very different from what has hitherto been believed, will be pleasant, and very fine. But that which will excite the greatest astonishment by far, and which indeed especially moved me to call the attention of all astronomers and philosophers, is this, namely, that I have discovered four planets, neither known nor observed by any one of the astronomers before my time, which have their orbits round a certain bright star, one of those previously known, like Venus and Mercury round the Sun, and are sometimes in front of it, sometimes behind it, though they never depart from it beyond certain limits. All which facts were discovered and observed a few days ago by the help of a telescope⁵ devised by me, through God’s grace first enlightening my mind.

Settling disputes about the Galaxy or Milky Way is by no means a minor achievement. Its nature is now clear to the senses themselves, not just to the understanding. Beyond this, showing the nature of certain stars as plainly as pointing with a finger will be pleasing and impressive. All astronomers until now have called these stars nebulous. I will demonstrate that their nature is very different from what people have previously believed. But one discovery will cause far greater astonishment than all the rest. This discovery especially moved me to call the attention of all astronomers and philosophers. I have discovered four planets that no astronomer before me had known or observed. These planets orbit a certain bright star, one already known, just as Venus and Mercury orbit the Sun. Sometimes they are in front of this star, sometimes behind it, though they never move beyond certain limits from it. I discovered and observed all these facts a few days ago with the help of a telescope⁵ I devised. God’s grace first enlightened my mind.

Perchance other discoveries still more excellent will be made from time to time by me or by other observers, with the assistance of a similar instrument, so I will first briefly record its shape and preparation, as well as the occasion of its being devised, and then I will give an account of the observations made by me.

Perhaps I or other observers will make even greater discoveries from time to time with a similar instrument. So I will first briefly describe the instrument’s shape, how to make it, and what led to its invention. Then I will describe my observations.

About ten months ago a report reached my ears that a Dutchman had constructed a telescope, by the aid of which visible objects, although at a great distance from the eye of the observer, were seen distinctly as if near; and some proofs of its most wonderful performances were reported, which some gave credence to, but others contradicted. A few days after, I received confirmation of the report in a letter written from Paris by a noble Frenchman, Jaques Badovere, which finally determined me to give myself up first to inquire into the principle of the telescope, and then to consider the means by which I might compass the invention of a similar instrument, which a little while after I succeeded in doing, through deep study of the theory of Refraction; and I prepared a tube, at first of lead, in the ends of which I fitted two glass lenses, both plane on one side, but on the other side one spherically convex, and the other concave. Then bringing my eye to the concave lens I saw objects satisfactorily large and near, for they appeared one-third of the distance off and nine times larger than when they are seen with the natural eye alone. I shortly afterwards constructed another telescope with more nicety, which magnified objects more than sixty times. At length, by sparing neither labour nor expense, I succeeded in constructing for myself an instrument so superior that objects seen through it appear magnified nearly a thousand times, and more than thirty times nearer than if viewed by the natural powers of sight alone.

About ten months ago, I heard a report that a Dutchman had built a telescope. Through this instrument, objects far from the observer’s eye could be seen clearly, as if they were nearby. Some evidence of its remarkable performance was reported. Some people believed these reports, but others disputed them. A few days later, I received confirmation in a letter from Paris, written by a French nobleman, Jaques Badovere. His letter finally persuaded me to investigate how the telescope worked. Then I considered how I might build a similar instrument. A short time later, I succeeded through careful study of the theory of refraction. I first made a lead tube and fitted a glass lens into each end. Both lenses were flat on one side. On the other side, one had a spherical outward curve, and the other curved inward. Looking through the concave lens, I saw objects enlarged and brought closer to a satisfactory degree. They appeared at one-third of their actual distance and nine times larger than when seen with the unaided eye. Soon afterward, I built another telescope with greater precision, which magnified objects more than sixty times. Finally, sparing neither effort nor expense, I succeeded in building myself a far superior instrument. Through this telescope, objects appear nearly a thousand times larger and more than thirty times closer than with unaided sight.

It would be altogether a waste of time to enumerate the number and importance of the benefits which this instrument may be expected to confer, when used by land or sea. But without paying attention to its use for terrestrial objects, I betook myself to observations of the heavenly bodies; and first of all, I viewed the Moon as near as if it was scarcely two semi-diameters⁶ of the Earth distant. After the Moon, I frequently observed other heavenly bodies, both fixed stars and planets, with incredible delight; and, when I saw their very great number, I began to consider about a method by which I might be able to measure their distances apart, and at length I found one. And here it is fitting that all who intend to turn their attention to observations of this kind should receive certain cautions. For, in the first place, it is absolutely necessary for them to prepare a most perfect telescope, one which will show very bright objects distinct and free from any mistiness, and will magnify them at least 400 times, for then it will show them as if only one-twentieth of their distance off. For unless the instrument be of such power, it will be in vain to attempt to view all the things which have been seen by me in the heavens, or which will be enumerated hereafter.

Listing the many important benefits this instrument may offer on land or sea would be a complete waste of time. But I set aside its use for viewing objects on Earth and turned to observing the heavenly bodies. First, I viewed the Moon as if it were scarcely two semi-diameters⁶ of the Earth away. After the Moon, I frequently observed other heavenly bodies, both fixed stars and planets, with incredible delight. When I saw their vast number, I began considering how I might measure the distances between them. At last, I found a method. Here, everyone planning to make observations of this kind should take certain precautions. First, they absolutely must prepare a telescope of the highest quality. The telescope must show very bright objects clearly, without any haziness, and magnify them at least 400 times. Objects will then appear as if they were only one-twentieth of their actual distance away. Without an instrument of such power, observers will try in vain to see all the things I have seen in the heavens. The same applies to all the things I will describe below.

But in order that any one may be a little more certain about the magnifying power of his instrument, he shall fashion two circles, or two square pieces of paper, one of which is 400 times greater than the other, but that will be when the diameter of the greater is twenty times the length of the diameter of the other. Then he shall view from a distance simultaneously both surfaces, fixed on the same wall, the smaller with one eye applied to the telescope, and the larger with the other eye unassisted; for that may be done without inconvenience at one and the same instant with both eyes open. Then both figures will appear of the same size, if the instrument magnifies objects in the desired proportion.

But to be a little more certain of your instrument’s magnifying power, make two circles or two square paper pieces. One should have 400 times the area of the other. For circles, the larger diameter should be twenty times the smaller diameter. Fix both shapes to the same wall and view them simultaneously from a distance. Look at the smaller shape through the telescope with one eye, and at the larger with the other, unaided eye. You can comfortably do both at once with both eyes open. The two shapes will appear the same size if the instrument magnifies objects in the desired proportion.

After such an instrument has been prepared, the method of measuring distances remains for inquiry, and this we shall accomplish by the following contrivance:—

Once such an instrument is ready, we still need to determine how to measure distances. We will do so using the following device:

For the sake of being more easily understood, I will suppose a tube A B C D.[^7] Let E be the eye of the observer; then, when there are no lenses in the tube rays from the eye to the object F G would be drawn in the straight lines E C F, E D G, but when the lenses have been inserted, let the rays go in the bent lines E C H, E D I,—for they are contracted, and those which originally, when unaffected by the lenses, were directed to the object F G, will include only the part H I. Hence the ratio of the distance E H to the line H I being known, we shall be able to find, by means of a table of sines, the magnitude of the angle subtended at the eye by the object H I, which we shall find to contain only some minutes. But if we fit on the lens C D thin plates of metal, pierced, some with larger, others with smaller apertures, by putting on over the lens sometimes one plate, sometimes another, as may be necessary, we shall construct at our pleasure different subtending angles of more or fewer minutes, by the help of which we shall be able to measure conveniently the intervals between stars separated by an angular distance of some minutes, within an error of one or two minutes. But let it suffice for the present to have thus slightly touched, and as it were just put our lips to these matters, for on some other opportunity I will publish the theory of this instrument in completeness.

To make the explanation easier to understand, I will imagine a tube A B C D.[^7] Let E be the observer’s eye. Without lenses in the tube, rays from the eye to the object F G would follow straight lines E C F and E D G. But with lenses inserted, let the rays follow the bent lines E C H and E D I. The lenses narrow the rays’ spread. Rays originally directed toward the object F G without lenses will now cover only the part H I. So, knowing the ratio of the distance E H to the line H I, we can use a table of sines. The table will let us find the angle that the object H I subtends at the eye. We will find that the angle measures only some minutes of arc. But suppose we fit thin metal plates over the lens C D, some pierced with larger openings, others with smaller ones. By placing one plate or another over the lens as needed, we can produce different subtended angles at will. These angles will measure more or fewer minutes of arc. With these angles, we can conveniently measure the distances between stars separated by an angular distance of some minutes. Our measurements will be accurate to within one or two minutes. But for now, let this brief discussion suffice, as though we have merely touched our lips to these matters. On another occasion, I will publish the complete theory of this instrument.

Now let me review the observations made by me during the two months just past, again inviting the attention of all who are eager for true philosophy to the beginnings which led to the sight of most important phenomena.

Now let me review my observations from the past two months. Once again, I invite all who seek true philosophy to consider the first steps that led to these important discoveries.

Let me speak first of the surface of the Moon, which is turned towards us. For the sake of being understood more easily, I distinguish two parts in it, which I call respectively the brighter and the darker. The brighter part seems to surround and pervade the whole hemisphere; but the darker part, like a sort of cloud, discolours the Moon’s surface and makes it appear covered with spots. Now these spots, as they are somewhat dark and of considerable size, are plain to every one, and every age has seen them, wherefore I shall call them great or ancient spots, to distinguish them from other spots, smaller in size, but so thickly scattered that they sprinkle the whole surface of the Moon, but especially the brighter portion of it. These spots have never been observed by any one before me; and from my observations of them, often repeated, I have been led to that opinion which I have expressed, namely, that I feel sure that the surface of the Moon is not perfectly smooth, free from inequalities and exactly spherical, as a large school of philosophers considers with regard to the Moon and the other heavenly bodies, but that, on the contrary, it is full of inequalities, uneven, full of hollows and protuberances, just like the surface of the Earth itself, which is varied everywhere by lofty mountains and deep valleys.

Let me begin with the surface of the Moon facing us. To make my meaning clearer, I divide this surface into two parts, which I call brighter and darker. The brighter part seems to surround and extend throughout the entire hemisphere. But the darker part discolors the Moon’s surface like a cloud, making it look spotted. These spots are somewhat dark and quite large, so everyone can see them, and people have seen them in every age. I will therefore call them great or ancient spots to distinguish them from other, smaller spots. These smaller spots are scattered so densely that they cover the Moon’s entire surface, especially the brighter part. No one before me has ever observed these smaller spots. My repeated observations have led me to the opinion I have already expressed. I am certain that the Moon’s surface is not perfectly smooth, free of irregularities, and exactly spherical. A large school of philosophers believes that the Moon and the other heavenly bodies have such surfaces. On the contrary, the Moon’s surface is uneven and full of hollows and raised areas. It is just like the Earth’s surface, which is varied everywhere by high mountains and deep valleys.

Sketches by Galileo to shew:—

Sketches by Galileo to show:

The appearances from which we may gather these conclusions are of the following nature:—On the fourth or fifth day after new-moon, when the Moon presents itself to us with bright horns, the boundary which divides the part in shadow from the enlightened part does not extend continuously in an ellipse, as would happen in the case of a perfectly spherical body, but it is marked out by an irregular, uneven, and very wavy line, as represented in the figure given, for several bright excrescences, as they may be called, extend beyond the boundary of light and shadow into the dark part, and on the other hand pieces of shadow encroach upon the light:—nay, even a great quantity of small blackish spots, altogether separated from the dark part, sprinkle everywhere almost the whole space which is at the time flooded with the Sun’s light, with the exception of that part alone which is occupied by the great and ancient spots. I have noticed that the small spots just mentioned have this common characteristic always and in every case, that they have the dark part towards the Sun’s position, and on the side away from the Sun they have brighter boundaries, as if they were crowned with shining summits. Now we have an appearance quite similar on the Earth about sunrise, when we behold the valleys, not yet flooded with light, but the mountains surrounding them on the side opposite to the Sun already ablaze with the splendour of his beams; and just as the shadows in the hollows of the Earth diminish in size as the Sun rises higher, so also these spots on the Moon lose their blackness as the illuminated part grows larger and larger. Again, not only are the boundaries of light and shadow in the Moon seen to be uneven and sinuous, but—and this produces still greater astonishment—there appear very many bright points within the darkened portion of the Moon, altogether divided and broken off from the illuminated tract, and separated from it by no inconsiderable interval, which, after a little while, gradually increase in size and brightness, and after an hour or two become joined on to the rest of the bright portion, now become somewhat larger; but in the meantime others, one here and another there, shooting up as if growing, are lighted up within the shaded portion, increase in size, and at last are linked on to the same luminous surface, now still more extended. An example of this is given in the same figure. Now, is it not the case on the Earth before sunrise, that while the level plain is still in shadow, the peaks of the most lofty mountains are illuminated by the Sun’s rays? After a little while does not the light spread further, while the middle and larger parts of those mountains are becoming illuminated; and at length, when the Sun has risen, do not the illuminated parts of the plains and hills join together? The grandeur, however, of such prominences and depressions in the Moon seems to surpass both in magnitude and extent the ruggedness of the Earth’s surface, as I shall hereafter show. And here I cannot refrain from mentioning what a remarkable spectacle I observed while the Moon was rapidly approaching her first quarter, a representation of which is given in the same illustration, placed opposite page 16. A protuberance of the shadow, of great size, indented the illuminated part in the neighbourhood of the lower cusp; and when I had observed this indentation longer, and had seen that it was dark throughout, at length, after about two hours, a bright peak began to arise a little below the middle of the depression; this by degrees increased, and presented a triangular shape, but was as yet quite detached and separated from the illuminated surface. Soon around it three other small points began to shine, until, when the Moon was just about to set, that triangular figure, having now extended and widened, began to be connected with the rest of the illuminated part, and, still girt with the three bright peaks already mentioned, suddenly burst into the indentation of shadow like a vast promontory of light.

The observations from which we may draw these conclusions are as follows. On the fourth or fifth day after the new Moon, the Moon appears with bright crescent tips. The boundary between shadow and light does not follow a continuous ellipse, as it would on a perfectly spherical body. Instead, the boundary forms an irregular, uneven, and very wavy line, as shown in the figure. Several bright projections, as we may call them, extend beyond the boundary into the dark part. Conversely, patches of shadow reach into the light. Indeed, many small, blackish spots are entirely separate from the dark part. These spots are scattered almost everywhere across the sunlit surface, except in the area occupied by the large, long-known spots. I have noticed that the small spots always share one feature, without exception. Their dark side faces the Sun, while their boundaries facing away from the Sun are brighter. The spots look as if they were crowned with shining peaks. We see something very similar on Earth around sunrise. The valleys are not yet filled with light. But the mountains surrounding them, on the side opposite the Sun, already blaze with the splendor of its rays. As the Sun rises higher, the shadows in Earth's hollows shrink. In the same way, these spots on the Moon lose their darkness as the illuminated area grows larger. Again, the boundary between light and shadow on the Moon is not merely uneven and winding. Even more astonishingly, many bright points appear within the Moon's darkened portion. These points are completely separate from the illuminated region, with a considerable gap between them and that region. After a little while, the points gradually grow larger and brighter. After an hour or two, they join the rest of the bright area, which has itself grown somewhat larger. Meanwhile, other points light up here and there within the shaded portion, springing up as if growing. These points also increase in size and finally join the same bright surface, which has now spread still farther. The same figure shows an example. Does not the same thing happen on Earth before sunrise? While the level plain remains in shadow, the Sun's rays light the highest mountain peaks. After a little while, does not the light spread farther, illuminating the middle and larger portions of those mountains? Finally, when the Sun has risen, do not the illuminated areas of the plains and hills join together? Yet the Moon's heights and hollows seem to surpass the rugged features of Earth's surface in both size and extent. I shall show this later. Here I cannot resist describing a remarkable sight I observed as the Moon rapidly approached its first quarter. The same illustration, opposite page 16, shows this sight. A large projection of shadow cut into the illuminated area near the lower crescent tip. I watched this indentation for some time and saw that it was dark throughout. At last, after about two hours, a bright peak began to appear a little below the middle of the depression. The peak gradually grew and took on a triangular shape, but remained entirely separate from the illuminated surface. Soon, three other small points began to shine around the peak. Then, just as the Moon was about to set, the triangular shape, now longer and wider, began joining the illuminated area. Still surrounded by the three bright peaks already mentioned, the triangle suddenly burst into the shadowed indentation like a vast promontory of light.

At the ends of the upper and lower cusps also certain bright points, quite away from the rest of the bright part, began to rise out of the shadow, as is seen depicted in the same illustration.

At the tips of the upper and lower cusps, some bright points also began to emerge from the shadow. These points were well separated from the rest of the bright area, as shown in the same illustration.

In both horns also, but especially in the lower one, there was a great quantity of dark spots, of which those which are nearer the boundary of light and shadow appear larger and darker, but those which are more remote less dark and more indistinct. In all cases, however, just as I have mentioned before, the dark portion of the spot faces the position of the Sun’s illumination, and a brighter edge surrounds the darkened spot on the side away from the Sun, and towards the region of the Moon in shadow. This part of the surface of the Moon, where it is marked with spots like a peacock’s tail with its azure eyes, is rendered like those glass vases which, through being plunged while still hot from the kiln into cold water, acquire a crackled and wavy surface, from which circumstance they are commonly called frosted glasses.⁸ Now the great spots of the Moon observed at the same time are not seen to be at all similarly broken, or full of depressions and prominences, but rather to be even and uniform; for only here and there some spaces, rather brighter than the rest, crop up; so that if any one wishes to revive the old opinion of the Pythagoreans, that the Moon is another Earth, so to say, the brighter portion may very fitly represent the surface of the land, and the darker the expanse of water. Indeed, I have never doubted that if the sphere of the Earth were seen from a distance, when flooded with the Sun’s rays, that part of the surface which is land would present itself to view as brighter, and that which is water as darker in comparison. Moreover, the great spots in the Moon are seen to be more depressed than the brighter tracts; for in the Moon, both when crescent and when waning, on the boundary between the light and shadow, which projects in some places round the great spots, the adjacent regions are always brighter, as I have noticed in drawing my illustrations, and the edges of the spots referred to are not only more depressed than the brighter parts, but are more even, and are not broken by ridges or ruggednesses. But the brighter part stands out most near the spots, so that both before the first quarter and about the third quarter also, around a certain spot in the upper part of the figure, that is, occupying the northern region of the Moon, some vast prominences on the upper and lower sides of it rise to an enormous elevation, as the illustrations show. This same spot before the third quarter is seen to be walled round with boundaries of a deeper shade, which just like very lofty mountain summits appear darker on the side away from the Sun, and brighter on the side where they face the Sun; but in the case of the cavities the opposite happens, for the part of them away from the Sun appears brilliant, and that part which lies nearer to the Sun dark and in shadow. After a time, when the enlightened portion of the Moon’s surface has diminished in size, as soon as the whole or nearly so of the spot already mentioned is covered with shadow, the brighter ridges of the mountains mount high above the shade. These two appearances are shown in the illustrations which are given.

Both horns of the crescent also had many dark spots, especially the lower horn. The spots nearer the boundary between light and shadow appear larger and darker. Those farther away appear less dark and less distinct. In every case, however, the dark part of the spot faces the Sun’s light, just as I mentioned before. A brighter edge surrounds the spot on the side away from the Sun, toward the Moon’s shadowed region. This spotted part of the Moon’s surface resembles a peacock’s tail with its blue eyes. It also resembles glass vases plunged into cold water while still hot from the kiln. This treatment gives the vases a cracked, wavy surface, so they are commonly called frosted glasses.⁸ The Moon’s large spots, observed at the same time, do not appear similarly broken or covered with hollows and rises. Instead, they appear level and uniform, with only a few patches here and there brighter than the rest. So someone might wish to revive the old Pythagorean view that the Moon is, so to speak, another Earth. In that case, the brighter part could very fittingly represent land, and the darker part an expanse of water. Indeed, I have never doubted how Earth’s sphere would look from a distance when flooded with the Sun’s rays. Its land would appear brighter, and its water darker by comparison. Moreover, the Moon’s large spots appear lower than the brighter areas. When the Moon is either a crescent or waning, the boundary between light and shadow extends around these large spots in places. The neighboring regions are always brighter, as I noticed while drawing my illustrations. The edges of these spots are not only lower than the brighter parts, but also smoother, without ridges or roughness. But the brighter ground rises highest near the spots. This happens both before the first quarter and around the third quarter. Around one spot in the upper part of the image, in the Moon’s northern region, vast heights rise above and below. These rise to an enormous height, as the illustrations show. Before the third quarter, this same spot appears surrounded by darker boundaries, like enclosing walls. Like very high mountain peaks, these boundaries appear darker on the side away from the Sun. They appear brighter on the side facing the Sun. But the opposite happens in hollows. The side away from the Sun appears brilliant, while the side nearer the Sun is dark and shadowed. Later, the sunlit part of the Moon’s surface shrinks. As soon as shadow covers all or nearly all of this spot, the brighter mountain ridges rise high above the shadow. The accompanying illustrations show these two appearances.

There is one other point which I must on no account forget, which I have noticed and rather wondered at. It is this:—The middle of the Moon, as it seems, is occupied by a certain cavity larger than all the rest, and in shape perfectly round. I have looked at this depression near both the first and third quarters, and I have represented it as well as I can in the second illustration already given. It produces the same appearance as to effects of light and shade as a tract like Bohemia would produce on the Earth, if it were shut in on all sides by very lofty mountains arranged on the circumference of a perfect circle; for the tract in the Moon is walled in with peaks of such enormous height that the furthest side adjacent to the dark portion of the Moon is seen bathed in sunlight before the boundary between light and shade reaches half-way across the circular space. But according to the characteristic property of the rest of the spots, the shaded portion of this too faces the Sun, and the bright part is towards the dark side of the Moon, which for the third time I advise to be carefully noticed as a most solid proof of the ruggednesses and unevennesses spread over the whole of the bright region of the Moon. Of these spots, moreover, the darkest are always those which are near to the boundary-line between the light and the shadow, but those further off appear both smaller in size and less decidedly dark; so that at length, when the Moon at opposition becomes full, the darkness of the cavities differs from the brightness of the prominences with a subdued and very slight difference.

There is one more point I must not forget, something I have noticed with some wonder. The middle of the Moon seems to contain a perfectly round hollow larger than all the others. I have observed this depression near both the first and third quarters. I have shown it as well as I can in the second illustration already given. Its light and shadow look as a region like Bohemia would look on Earth under the following conditions. Very high mountains would have to surround the region on all sides, forming a perfect circle. The Moon’s hollow is enclosed by peaks so high that sunlight reaches the far side beside the Moon’s dark portion before the boundary between light and shadow crosses half the circle. But, as with the other spots, the shaded part of this hollow faces the Sun. Its bright part faces the Moon’s dark side. For the third time, I urge careful attention to this pattern. It is very strong evidence of roughness and unevenness throughout the Moon’s bright region. Moreover, the darkest spots are always those near the boundary between light and shadow. Spots farther from that boundary appear both smaller and less distinctly dark. So, finally, when the Moon is opposite the Sun and becomes full, the contrast is faint and very slight. The darkness of the hollows then differs only slightly from the brightness of the raised areas.

These phenomena which we have reviewed are observed in the bright tracts of the Moon. In the great spots we do not see such differences of depressions and prominences as we are compelled to recognise in the brighter parts, owing to the change of their shapes under different degrees of illumination by the Sun’s rays according to the manifold variety of the Sun’s position with regard to the Moon. Still, in the great spots there do exist some spaces rather less dark than the rest, as I have noted in the illustrations, but these spaces always have the same appearance, and the depth of their shadow is neither intensified nor diminished; they do appear indeed sometimes a little more shaded, sometimes a little less, but the change of colour is very slight, according as the Sun’s rays fall upon them more or less obliquely; and besides, they are joined to the adjacent parts of the spots with a very gradual connection, so that their boundaries mingle and melt into the surrounding region. But it is quite different with the spots which occupy the brighter parts of the Moon’s surface, for, just as if they were precipitous crags with numerous rugged and jagged peaks, they have well-defined boundaries through the sharp contrast of light and shade. Moreover, inside those great spots certain other tracts are seen brighter than the surrounding region, and some of them very bright indeed, but the appearance of these, as well as of the darker parts, is always the same; there is no change of shape or brightness or depth of shadow, so that it becomes a matter of certainty and beyond doubt that their appearance is owing to real dissimilarity of parts, and not to unevennesses only in their configuration, changing in different ways the shadows of the same parts according to the variations of their illumination by the Sun, which really happens in the case of the other smaller spots occupying the brighter portion of the Moon, for day by day they change, increase, decrease, or disappear, inasmuch as they derive their origin only from the shadows of prominences.

The phenomena we have reviewed are seen in the Moon’s bright regions. In the large dark patches, we do not see the same differences between hollows and raised areas. We must recognize these features in the brighter regions because their shapes change under different degrees of sunlight. These changes depend on the many different positions of the Sun relative to the Moon. Still, some areas within the large dark patches are less dark than the rest, as I have shown in the illustrations. But these areas always look the same, and their shadows neither deepen nor fade. They do sometimes appear slightly more shaded or slightly less so. But this change in color is very slight, depending on how steeply the Sun’s rays strike them. These areas also blend very gradually into the neighboring parts of the dark patches. Their boundaries merge and fade into the surrounding region. But the spots in the brighter parts of the Moon’s surface are quite different. These spots have clearly defined boundaries because of the sharp contrast between light and shade. They look just as if they were steep crags with many rough, jagged peaks. Within the large dark patches, other areas appear brighter than their surroundings, and some are very bright indeed. But these brighter areas, like the darker ones, always look the same. Their shape, brightness, and depth of shadow do not change. We can therefore be certain, beyond doubt, that their appearance comes from real differences between parts of the surface. It does not come merely from uneven terrain casting different shadows as the sunlight changes. That shifting of shadows does happen with the smaller spots in the Moon’s brighter regions. Day by day, these smaller spots change, grow, shrink, or disappear, because they come only from shadows cast by raised features.

But here I feel that some people may be troubled with grave doubt, and perhaps seized with a difficulty so serious as to compel them to feel uncertain about the conclusion just explained and supported by so many phenomena. For if that part of the Moon’s surface which reflects the Sun’s rays most brightly is full of sinuosities, protuberances, and cavities innumerable, why, when the Moon is increasing, does the outer edge which looks toward the west, when the Moon is waning, the other half-circumference towards the east, and at full-moon the whole circle, appear not uneven, rugged, and irregular, but perfectly round and circular, as sharply defined as if marked out with a pair of compasses, and without the indentations of any protuberances or cavities? And most remarkably so, because the whole unbroken edge belongs to that part of the Moon’s surface which possesses the property of appearing brighter than the rest, which I have said to be throughout full of protuberances and cavities. For not one of the Great Spots extends quite to the circumference, but all of them are seen to be together away from the edge. Of this phenomenon, which affords a handle for such serious doubt, I produce two causes, and so two solutions of the difficulty.

But here I feel that some people may have serious doubts. Perhaps they may find a difficulty so great that they question the conclusion just explained and supported by so many observations. For suppose the Moon’s brightest surface, which reflects the Sun’s rays most strongly, has countless winding features, bumps, and hollows. Why, then, does the Moon’s outer edge look smooth rather than uneven, rough, and irregular? As the Moon waxes, the western edge looks smooth. As the Moon wanes, the eastern half of its outline looks smooth. At full moon, the entire outline looks smooth. Why does the edge appear perfectly round, as sharply defined as if drawn with a pair of compasses? Why are there no indentations caused by bumps or hollows? The smooth outline is especially remarkable because the entire unbroken edge belongs to the brighter part of the Moon’s surface. Yet I have said that this brighter surface is covered throughout with bumps and hollows. For not one of the Great Spots reaches the outer edge. All the Great Spots lie away from the edge. I offer two causes for this appearance, which raises such serious doubts, and thus two solutions to the difficulty.

The first solution which I offer is this:—If the protuberances and cavities in the body of the Moon existed only on the edge of the circle that bounds the hemisphere which we see, then the Moon might, or rather must, show itself to us with the appearance of a toothed wheel, being bounded with an irregular and uneven circumference; but if, instead of a single set of prominences arranged along the actual circumference only, very many ranges of mountains with their cavities and ruggednesses are set one behind the other along the extreme edge of the Moon, and that too not only in the hemisphere which we see, but also in that which is turned away from us, but still near the boundary of the hemisphere, then the eye, viewing them afar off, will not at all be able to detect the differences of prominences and cavities, for the intervals between the mountains situated in the same circle, or in the same chain, are hidden by the jutting forward of other prominences situated in other ranges, and especially if the eye of the observer is placed in the same line with the tops of the prominences mentioned. So on the Earth, the summits of a number of mountains close together appear situated in one plane, if the spectator is a long way off and standing at the same elevation. So when the sea is rough, the tops of the waves seem to form one plane, although between the billows there is many a gulf and chasm, so deep that not only the hulls, but even the bulwarks, masts, and sails of stately ships are hidden amongst them. Therefore, as within the Moon, as well as round her circumference, there is a manifold arrangement of prominences and cavities, and the eye, regarding them from a great distance, is placed in nearly the same plane with their summits, no one need think it strange that they present themselves to the visual ray which just grazes them as an unbroken line quite free from unevennesses. To this explanation may be added another, namely, that there is round the body of the Moon, just as round the Earth, an envelope of some substance denser than the rest of the ether, which is sufficient to receive and reflect the Sun’s rays, although it does not possess so much opaqueness as to be able to prevent our seeing through it—especially when it is not illuminated. That envelope, when illuminated by the Sun’s rays, renders the body of the Moon apparently larger than it really is, and would be able to stop our sight from penetrating to the solid body of the Moon, if its thickness were greater; now, it is of greater thickness about the circumference of the Moon, greater, I mean, not in actual thickness, but with reference to our sight-rays, which cut it obliquely; and so it may stop our vision, especially when it is in a state of brightness, and may conceal the true circumference of the Moon on the side towards the Sun.

The first explanation I offer is this: suppose the Moon’s raised areas and hollows existed only along the visible hemisphere’s edge. Then the Moon might, or rather must, look like a toothed wheel, with an irregular, uneven outline. But suppose many mountain ranges, with their hollows and rugged slopes, stand one behind another along the Moon’s outer edge. Suppose these ranges lie not only on the visible hemisphere, but also near the boundary on the hemisphere facing away. Then, from a great distance, the eye could not detect the differences between raised areas and hollows. The gaps between mountains in the same circle or chain would be hidden by projecting peaks in other ranges. This would be especially true if the observer’s eye were in line with the mountain tops. Likewise, on Earth, the summits of closely grouped mountains appear to lie in one plane to a distant observer at the same height. So too, when the sea is rough, the wave tops seem to form a single plane. Yet between the waves are many deep gulfs and chasms. These hide not only the hulls of grand ships, but even their bulwarks, masts, and sails. The Moon likewise has many raised areas and hollows, both across its surface and around its edge. From our great distance, our eyes are almost level with their summits. So no one should find it strange that these features appear as an unbroken, smooth line when viewed along their tops. Another explanation may be added. The Moon, like the Earth, is surrounded by a layer of some substance denser than the rest of the ether. This layer is dense enough to receive and reflect the Sun’s rays. Yet it is not opaque enough to prevent us from seeing through it, especially when it is not illuminated. When lit by the Sun, this layer makes the Moon appear larger than it really is. If the layer were thicker, it could prevent us from seeing the Moon’s solid body. Now, the layer is thicker around the Moon’s edge, though not in its actual depth. Rather, our lines of sight pass through it at an angle, crossing more of the layer. So the layer may block our view, especially when brightly lit. It may therefore hide the Moon’s true outline on the side facing the Sun.

This may be understood more clearly from the adjoining figure, in which the body of the Moon, A B C, is surrounded by an enveloping atmosphere, D E G. An eye at F penetrates to the middle parts of the Moon, as at A, through a thickness, D A, of the atmosphere; but towards the extreme parts a mass of atmosphere of greater depth, E B, shuts out its boundary from our sight. An argument in favour of this is, that the illuminated portion of the Moon appears of larger circumference than the rest of the orb which is in shadow.

The accompanying figure may make this clearer. The Moon’s body, A B C, is surrounded by an atmosphere, D E G. An eye at F sees the Moon’s central parts, such as A, through a thickness of atmosphere, D A. But near the Moon’s edges, a deeper layer of atmosphere, E B, hides the Moon’s boundary from view. One argument supporting this explanation is that the Moon’s illuminated portion appears to have a larger circumference than the portion in shadow.

Perhaps also some will think that this same cause affords a very reasonable explanation why the greater spots on the Moon are not seen to reach to the edge of the circumference on any side, although it might be expected that some would be found about the edge as well as elsewhere; and it seems credible that there are spots there, but that they cannot be seen because they are hidden by a mass of atmosphere too thick and too bright for the sight to penetrate.

Perhaps some will also think this same cause reasonably explains why the Moon’s larger spots never appear to reach its edge. Yet one might expect to find some spots along the edge as well as elsewhere. It seems plausible that spots exist there but cannot be seen because a mass of atmosphere hides them. This atmosphere is too thick and too bright for us to see through.

I think that it has been sufficiently made clear, from the explanation of phenomena which have been given, that the brighter part of the Moon’s surface is dotted everywhere with protuberances and cavities; it only remains for me to speak about their size, and to show that the ruggednesses of the Earth’s surface are far smaller than those of the Moon’s; smaller, I mean, absolutely, so to say, and not only smaller in proportion to the size of the orbs on which they are. And this is plainly shown thus:—As I often observed in various positions of the Moon with reference to the Sun, that some summits within the portion of the Moon in shadow appeared illumined, although at some distance from the boundary of the light (the terminator), by comparing their distance with the complete diameter of the Moon, I learnt that it sometimes exceeded the one-twentieth (1/20th) part of the diameter. Suppose the distance to be exactly 1/20th part of the diameter, and let the diagram represent the Moon’s orb, of which C A F is a great circle, E its centre, and C F a diameter, which consequently bears to the diameter of the Earth the ratio 2:7; and since the diameter of the Earth, according to the most exact observations, contains 7000 Italian miles, C F will be 2000, and C E 1000, and the 1/20th part of the whole, C F, 100 miles. Also let C F be a diameter of the great circle which divides the bright part of the Moon from the dark part (for, owing to the very great distance of the Sun from the Moon this circle does not differ sensibly from a great one), and let the distance of A from the point C be 1/20th part of that diameter; let the radius E A be drawn, and let it be produced to cut the tangent line G C D, which represents the ray that illumines the summit, in the point D. Then the arc C A or the straight line C D will be 100 of such units, as C E contains 1000. The sum of the squares of D C, C E is therefore 1,010,000, and the square of D E is equal to this; therefore the whole E D will be more than 1004; and A D will be more than 4 of such units, as C E contained 1000. Therefore the height of A D in the Moon, which represents a summit reaching up to the Sun’s ray, G C D, and separated from the extremity C by the distance C D, is more than 4 Italian miles; but in the Earth there are no mountains which reach to the perpendicular height even of one mile. We are therefore left to conclude that it is clear that the prominences of the Moon are loftier than those of the Earth.

I think the explanations already given make one point sufficiently clear. The brighter part of the Moon’s surface is dotted everywhere with raised areas and hollows. I now need only discuss their size and show that Earth’s surface features are far smaller than the Moon’s. I mean smaller in absolute terms, so to speak, not just relative to the size of each globe. The evidence is plain. I often observed the Moon in various positions relative to the Sun. Some peaks in the Moon’s shadowed portion appeared lit, although they lay some distance from the boundary of light (the terminator). I compared their distance from that boundary with the Moon’s full diameter. I found that the distance sometimes exceeded one-twentieth (1/20) of the diameter. Suppose the distance is exactly 1/20 of the diameter. Let the diagram represent the Moon’s globe, with C A F as a great circle and E as its center. Let C F be a diameter, whose ratio to Earth’s diameter is therefore 2:7. According to the most exact observations, Earth’s diameter is 7000 Italian miles. So C F will be 2000 miles, C E will be 1000, and 1/20 of C F will be 100 miles. Also, let C F be a diameter of the great circle dividing the Moon’s bright part from its dark part. Because the Sun is so far from the Moon, this dividing circle differs imperceptibly from a great circle. Let the distance from A to C be 1/20 of that diameter. Draw the radius E A and extend it to meet the tangent line G C D at D. The line G C D represents the ray lighting the peak. Then the arc C A or the straight line C D will measure 100 units, while C E measures 1000. The sum of the squares of D C and C E is therefore 1,010,000. The square of D E equals this sum. Therefore, the full length E D will be more than 1004 units. A D will thus be more than 4 units, on the same scale where C E measures 1000. On the Moon, A D represents the height of a peak reaching the Sun’s ray, G C D. The peak is separated from the boundary point C by the distance C D. The peak’s height is therefore more than 4 Italian miles. But on Earth, no mountains reach even one mile in vertical height. We must therefore conclude that the Moon’s peaks are clearly higher than Earth’s.

I wish in this place to assign the cause of another lunar phenomenon well worthy of notice, and although this phenomenon was observed by me not lately, but many years ago, and has been pointed out to some of my intimate friends and pupils, explained, and assigned to its true cause, yet as the observation of it is rendered easier and more vivid by the help of a telescope, I have considered that it would not be unsuitably introduced in this place, but I wish to introduce it chiefly in order that the connection and resemblance between the Moon and the Earth may appear more plainly.

I want to explain here the cause of another lunar phenomenon well worth noting. Although I observed this phenomenon many years ago, rather than recently, a telescope makes it easier and clearer to observe. I have already shown it to some of my close friends and pupils and explained its true cause. Because a telescope improves the view, I think this is a suitable place to discuss the phenomenon. But my main purpose is to make the connection and resemblance between the Moon and the Earth clearer.

When the Moon, both before and after conjunction, is found not far from the Sun, not only does its orb show itself to our sight on the side where it is furnished with shining horns, but a slight and faint circumference is also seen to mark out the circle of the dark part, that part, namely, which is turned away from the Sun, and to separate it from the darker background of the sky. But if we examine the matter more closely, we shall see that not only is the extreme edge of the part in shadow shining with a faint brightness, but that the entire face of the Moon, that side, that is, which does not feel the Sun’s glare, is illuminated with a pale light of considerable brightness. At the first glance only a fine circumference appears shining, on account of the darker part of the sky adjacent to it; whereas, on the contrary, the rest of the surface appears dark, on account of the contiguity of the shining horns, which destroys the clearness of our sight. But if any one chooses such a position for himself, that by the interposition of a roof, or a chimney, or some other object between his sight and the Moon, but at a considerable distance from his eye, the shining horns are hidden, and the rest of the Moon’s orb is left exposed to his view, then he will find that this tract of the Moon also, although deprived of sunlight, gleams with considerable light, and particularly so if the gloom of the night has already deepened through the absence of the Sun; for with a darker background the same light appears brighter. Moreover, it is found that this secondary brightness of the Moon, as I may call it, is greater in proportion as the Moon is less distant from the Sun; for it abates more and more in proportion to the Moon’s distance from that body, so much so that after the first quarter, and before the end of the second, it is found to be weak and very faint, although it be observed in a darker sky; whereas, at an angular distance of 60° or less, even during twilight, it is wonderfully bright, so bright indeed that, with the help of a good telescope, the great spots may be distinguished in it.

When the Moon is near the Sun, both before and after conjunction, we see more than its bright crescent. A faint outline also marks the edge of the dark part, the part turned away from the Sun. This outline separates the Moon from the darker sky behind it. But if we look more closely, we see that more than the outer edge of the shadowed part shines faintly. The entire face of the Moon untouched by the Sun’s glare glows with a pale light of considerable brightness. At first glance, only a thin outline seems to shine, because the sky beside it is darker. The rest of the surface, by contrast, seems dark because the nearby bright crescent interferes with our vision. But an observer can choose a position where a roof, chimney, or another object blocks the bright crescent. The object must stand well away from the observer’s eye and leave the rest of the Moon visible. Then the observer will find that even the part without sunlight glows with considerable light. This glow is especially clear if the night has already grown darker in the Sun’s absence. Against a darker background, the same light looks brighter. Moreover, this secondary brightness of the Moon, as I may call it, grows stronger as the Moon approaches the Sun. The brightness fades more and more as the Moon moves farther from the Sun. After the first quarter and before the end of the second, the glow is weak and very faint. This is true even when the Moon is observed in a darker sky. Yet at an angular distance of 60° or less from the Sun, the glow is wonderfully bright, even during twilight. Indeed, with a good telescope, the large spots can be distinguished in this glow.

This strange brightness has afforded no small perplexity to philosophical minds; and some have published one thing, some another, as the cause to be alleged for it. Some have said that it is the inherent and natural brightness of the Moon; some that it is imparted to that body by the planet Venus; or, as others maintain, by all the stars; while some have said that it comes from the Sun, whose rays, they say, find a way through the solid mass of the Moon. But statements of this kind are disproved without much difficulty, and convincingly demonstrated to be false. For if this kind of light were the Moon’s own, or were contributed by the stars, the Moon would retain it, particularly in eclipses, and would show it then, when left in an unusually dark sky, but this is contrary to experience. For the brightness which is seen on the Moon in eclipses is far less intense, being somewhat reddish, and almost copper-coloured, whereas this is brighter and whiter; besides, the brightness seen during an eclipse is changeable and shifting, for it wanders over the face of the Moon, so that that part which is near the circumference of the circle of shadow thrown by the Earth is bright, but the rest of the Moon is always seen to be dark. From which circumstance we understand without hesitation that this brightness is due to the proximity of the Sun’s rays coming into contact with some denser region which surrounds the Moon as an envelope; owing to which contact a sort of dawn-light is diffused over the neighbouring regions of the Moon, just as the twilight spreads in the morning and evening on the Earth:¹¹ but I will treat more fully of this matter in my book upon the System of the Universe.¹²

This strange brightness has greatly puzzled philosophical thinkers. Some have published one explanation for its cause, and some another. Some have said that the brightness is natural to the Moon itself. Some say that the planet Venus gives the Moon this light, while others attribute it to all the stars. Still others say that the light comes from the Sun. They claim that the Sun’s rays find a way through the Moon’s solid mass. But such claims are easily disproved and convincingly shown to be false. For if this light belonged to the Moon itself, or came from the stars, the Moon would retain it, particularly during eclipses. The Moon would show this light then, against an unusually dark sky, but experience shows otherwise. For the brightness seen on the Moon during eclipses is much fainter, somewhat reddish, and almost copper-coloured. The brightness discussed here, however, is brighter and whiter. Besides, the brightness seen during an eclipse changes and shifts across the Moon’s face. The part near the edge of the Earth’s circular shadow is bright, while the rest of the Moon always appears dark. From this, we can conclude without hesitation that the eclipse brightness comes from nearby sunlight touching a denser region surrounding the Moon. This region surrounds the Moon like an envelope. Contact with the Sun’s rays spreads a kind of dawn light over the neighbouring regions of the Moon. This happens just as twilight spreads over the Earth in the morning and evening:¹¹ but I will discuss this matter more fully in my book on the System of the Universe.¹²

Again, to assert that this sort of light is imparted to the Moon by the planet Venus is so childish as to be undeserving of an answer; for who is so ignorant as not to understand that at conjunction and within an angular distance of 60° it is quite impossible for the part of the Moon turned away from the Sun to be seen by the planet Venus?

Again, claiming that the planet Venus gives the Moon this kind of light is childish beyond deserving an answer. For who could be so ignorant as not to understand the following? At conjunction and within an angular distance of 60°, Venus cannot possibly see the Moon’s side facing away from the Sun.

But that this light is derived from the Sun penetrating with its light the solid mass of the Moon, and rendering it luminous, is equally untenable. For then this light would never lessen, since the hemisphere of the Moon is always illumined by the Sun, except at the moment of a lunar eclipse, yet really it quickly decreases while the Moon is drawing near to the end of her first quarter, and when she has passed her first quarter it becomes quite dull. Since, therefore, this kind of secondary brightness is not inherent and the Moon’s own, nor borrowed from any of the stars, nor from the Sun, and since there now remains in the whole universe no other body whatever except the Earth, what, pray, must we conclude? What must we assert? Shall we assert that the body of the Moon, or some other dark and sunless orb, receives light from the Earth? Why should it not be the Moon? And most certainly it is. The Earth, with fair and grateful exchange, pays back to the Moon an illumination like that which it receives from the Moon nearly the whole time during the darkest gloom of night. Let me explain the matter more clearly. At conjunction, when the Moon occupies a position between the Sun and the Earth, the Moon is illuminated by the Sun’s rays on her half towards the Sun which is turned away from the Earth, and the other half, with which she regards the Earth, is covered with darkness, and so in no degree illumines the Earth’s surface. When the Moon has slightly separated from the Sun, straightway she is partly illumined on the half directed towards us; she turns towards us a slender silvery crescent, and slightly illumines the Earth; the Sun’s illumination increases upon the Moon as she approaches her first quarter, and the reflexion of that light increases on the Earth; the brightness in the Moon next extends beyond the semicircle, and our nights grow brighter; at length the entire face of the Moon looking towards the Earth is irradiated with the most intense brightness by the Sun, which happens when the Sun and Moon are on opposite sides of the Earth; then far and wide the surface of the Earth shines with the flood of moonlight; after this the Moon, now waning, sends out less powerful beams, and the Earth is illumined less powerfully; at length the Moon draws near her first position of conjunction with the Sun, and forthwith black night invades the Earth. In such a cycle the moonlight gives us each month alternations of brighter and fainter illumination. But the benefit of her light to the Earth is balanced and repaid by the benefit of the light of the Earth to her; for while the Moon is found near the Sun about the time of conjunction, she has in front of her the entire surface of that hemisphere of the Earth which is exposed to the Sun, and vividly illumined with his beams, and so receives light reflected from the Earth. Owing to such reflexion, the hemisphere of the Moon nearer to us, though deprived of sunlight, appears of considerable brightness. Again, when removed from the Sun through a quadrant, the Moon sees only one-half of the Earth’s hemisphere illuminated, namely the western half, for the other, the eastern, is covered with the shades of night; the Moon is, therefore, less brightly enlightened by the Earth, and accordingly that secondary light appears fainter to us. But if you imagine the Moon to be set on the opposite side of the Earth to the Sun, she will see the hemisphere of the Earth, now between the Moon and the Sun, quite dark, and steeped in the gloom of night; if, therefore, an eclipse should accompany such a position of the Moon, she will receive no light at all, being deprived of the illumination of the Sun and Earth together. In any other position, with regard to the Earth and the Sun, the Moon receives more or less light by reflexion from the Earth, according as she sees a greater or smaller portion of the hemisphere of the Earth illuminated by the Sun; for such a law is observed between these two orbs, that at whatever times the Earth is most brightly enlightened by the Moon, at those times, on the contrary, the Moon is least enlightened by the Earth; and contrariwise. Let these few words on this subject suffice in this place; for I will consider it more fully in my System of the Universe, where, by very many arguments and experimental proofs, there is shown to be a very strong reflexion of the Sun’s light from the Earth, for the benefit of those who urge that the Earth must be separated from the starry host, chiefly for the reason that it has neither motion nor light, for I will prove that the Earth has motion, and surpasses the Moon in brightness, and is not the place where the dull refuse of the universe has settled down; and I will support my demonstration by a thousand arguments taken from natural phenomena.

But the idea that this light comes from sunlight passing through the Moon’s solid body and making it glow is equally unsound. If that were true, this light would never fade. One hemisphere of the Moon is always lit by the Sun, except during a lunar eclipse. Yet this secondary light quickly decreases as the Moon approaches the end of its first quarter. After the Moon passes its first quarter, the light becomes quite faint. This secondary brightness, then, does not belong to the Moon itself. Nor does it come from any of the stars or from the Sun. And since no other body remains in the whole universe except the Earth, what, then, must we conclude? What must we say? Shall we say that the Moon, or some other dark orb without sunlight, receives light from the Earth? Why should it not be the Moon? And most certainly it is. In a fair and welcome exchange, the Earth gives light back to the Moon. This light resembles the moonlight the Earth receives through nearly all the darkest hours of night. Let me explain more clearly. At conjunction, the Moon stands between the Sun and the Earth. The Sun’s rays light the half facing the Sun, which faces away from the Earth. The other half faces the Earth and remains dark, so it gives no light at all to the Earth’s surface. As soon as the Moon moves a little away from the Sun, part of the half facing us becomes lit. The Moon presents a slender silver crescent to us and gives the Earth a little light. As the Moon approaches its first quarter, sunlight covers more of its face. More of that light is then reflected onto the Earth. Next, the bright part of the Moon extends beyond a semicircle, and our nights grow brighter. Finally, the Sun lights the Moon’s entire Earth-facing side with the greatest intensity. This happens when the Sun and Moon stand on opposite sides of the Earth. Then the Earth’s surface shines far and wide under a flood of moonlight. Afterward, as the Moon wanes, its rays grow weaker, and the Earth receives less light. Finally, the Moon returns toward its original position in conjunction with the Sun, and black night immediately overtakes the Earth. Through this monthly cycle, moonlight alternates between brighter and fainter illumination. But the Earth repays the benefit of moonlight by giving its own light to the Moon. Near conjunction, when the Moon is close to the Sun, the Moon faces the Earth’s entire sunlit hemisphere. That hemisphere shines brightly in the Sun’s rays, so the Moon receives light reflected from the Earth. Because of this reflection, the Moon’s near side appears quite bright, though it receives no sunlight. Again, when the Moon has moved a quarter-circle away from the Sun, it sees only half the Earth’s hemisphere illuminated. The western half is lit, while the eastern half lies in the darkness of night. The Moon therefore receives less light from the Earth, and its secondary light accordingly appears fainter to us. But imagine the Moon on the opposite side of the Earth from the Sun. The Earth now stands between the Moon and the Sun. The hemisphere facing the Moon appears completely dark, steeped in the gloom of night. If an eclipse occurs while the Moon is in this position, the Moon will receive no light at all. It will be deprived of light from both the Sun and the Earth. In any other position relative to the Earth and Sun, the Moon receives more or less reflected light from the Earth. The amount depends on how much of the Earth’s sunlit hemisphere the Moon can see. For these two bodies follow this rule: when moonlight illuminates the Earth most brightly, earthlight illuminates the Moon least brightly. And the reverse is also true. Let these few words on the subject suffice here. I will examine the matter more fully in my System of the Universe. There, many arguments and experimental proofs show that the Earth reflects the Sun’s light very strongly. This demonstration is intended for those who argue that the Earth must be excluded from the company of the stars. Their chief reason is that the Earth has neither motion nor light. For I will prove that the Earth moves and surpasses the Moon in brightness. I will also prove that the Earth is not where the universe’s dull waste has settled. And I will support my demonstration with a thousand arguments drawn from natural phenomena.

Hitherto I have spoken of the observations which I have made concerning the Moon’s body; now I will briefly announce the phenomena which have been, as yet, seen by me with reference to the Fixed Stars. And first of all the following fact is worthy of consideration:—The stars, fixed as well as erratic, when seen with a telescope, by no means appear to be increased in magnitude in the same proportion as other objects, and the Moon herself, gain increase of size; but in the case of the stars such increase appears much less, so that you may consider that a telescope, which (for the sake of illustration) is powerful enough to magnify other objects a hundred times, will scarcely render the stars magnified four or five times. But the reason of this is as follows:—When stars are viewed with our natural eyesight they do not present themselves to us of their bare, real size, but beaming with a certain vividness, and fringed with sparkling rays, especially when the night is far advanced; and from this circumstance they appear much larger than they would if they were stripped of those adventitious fringes, for the angle which they subtend at the eye is determined not by the primary disc of the star, but by the brightness which so widely surrounds it. Perhaps you will understand this most clearly from the well-known circumstance that when stars rise just at sunset, in the beginning of twilight, they appear very small, although they may be stars of the first magnitude; and even the planet Venus itself, on any occasion when it may present itself to view in broad daylight, is so small to see that it scarcely seems to equal a star of the last magnitude. It is different in the case of other objects, and even of the Moon, which, whether viewed in the light of midday or in the depth of night, always appears of the same size. We conclude therefore that the stars are seen at midnight in uncurtailed glory, but their fringes are of such a nature that the daylight can cut them off, and not only daylight, but any slight cloud which may be interposed between a star and the eye of the observer. A dark veil or coloured glass has the same effect, for, upon placing them before the eye between it and the stars, all the blaze that surrounds them leaves them at once. A telescope also accomplishes the same result, for it removes from the stars their adventitious and accidental splendours before it enlarges their true discs (if indeed they are of that shape), and so they seem less magnified than other objects, for a star of the fifth or sixth magnitude seen through a telescope is shown as of the first magnitude only.

So far, I have described my observations of the Moon’s body. Now I will briefly report what I have seen so far concerning the Fixed Stars. First, one fact deserves attention. Through a telescope, stars, both fixed and wandering, do not appear enlarged as much as other objects or the Moon. The increase in their apparent size is much smaller. For example, a telescope powerful enough to magnify other objects a hundred times will scarcely magnify stars four or five times. The reason is as follows. To the naked eye, stars do not appear at their bare, true size. Instead, they shine brightly and are fringed with sparkling rays, especially late at night. These fringes make stars appear much larger than they would without them. The angle a star occupies in our vision depends not on its central disc, but on the surrounding glow. Perhaps a familiar observation will make this clearest. When stars rise just after sunset, at the beginning of twilight, they appear very small, even stars of the first magnitude. Even the planet Venus, whenever visible in broad daylight, looks scarcely as large as a star of the faintest magnitude. Other objects are different. Even the Moon always appears the same size, whether seen at midday or in the deepest night. We therefore conclude that stars appear at midnight in their full glory. But daylight can strip away their fringes. Even a slight cloud between a star and the observer’s eye can remove these fringes. A dark veil or coloured glass has the same effect. When either is placed between the eye and the stars, all the surrounding blaze immediately disappears. A telescope does the same thing. Before enlarging the stars’ true discs, if they are indeed disc-shaped, the telescope removes the extra brilliance surrounding them. Stars therefore seem less magnified than other objects. A star of the fifth or sixth magnitude, seen through a telescope, appears only as a star of the first magnitude.

The difference between the appearance of the planets and the fixed stars seems also deserving of notice. The planets present their discs perfectly round, just as if described with a pair of compasses, and appear as so many little moons, completely illuminated and of a globular shape; but the fixed stars do not look to the naked eye bounded by a circular circumference, but rather like blazes of light, shooting out beams on all sides and very sparkling, and with a telescope they appear of the same shape as when they are viewed by simply looking at them, but so much larger that a star of the fifth or sixth magnitude seems to equal Sirius, the largest of all the fixed stars.¹³

The difference in appearance between planets and fixed stars also seems worth noting. The planets show perfectly round discs, just as if drawn with a compass. They look like little moons, fully lit and spherical. But to the naked eye, fixed stars do not appear to have circular outlines. Instead, they look like intensely sparkling blazes of light, sending out rays in all directions. Through a telescope, fixed stars have the same shape as when viewed with the naked eye, but appear much larger. A star of the fifth or sixth magnitude then seems to match Sirius, the largest of all the fixed stars.¹³

But beyond the stars of the sixth magnitude you will behold through the telescope a host of other stars, which escape the unassisted sight, so numerous as to be almost beyond belief, for you may see more than six other differences of magnitude, and the largest of these, which I may call stars of the seventh magnitude, or of the first magnitude of invisible stars, appear with the aid of the telescope larger and brighter than stars of the second magnitude seen with the unassisted sight. But in order that you may see one or two proofs of the inconceivable manner in which they are crowded together, I have determined to make out a case against two star-clusters, that from them as a specimen you may decide about the rest.

But beyond the sixth-magnitude stars, the telescope will reveal a host of other stars invisible to the naked eye. Their numbers are almost beyond belief, for you can see more than six additional levels of magnitude. The largest of these I may call seventh-magnitude stars, or the first magnitude of invisible stars. Through the telescope, these appear larger and brighter than second-magnitude stars seen with the naked eye. But I want to give you one or two examples of how inconceivably crowded these stars are. For this purpose, I have decided to present evidence from two star clusters. From these examples, you may judge the rest.

As my first example I had determined to depict the entire constellation of Orion, but I was overwhelmed by the vast quantity of stars and by want of time, and so I have deferred attempting this to another occasion, for there are adjacent to, or scattered among, the old stars more than five hundred new stars within the limits of one or two degrees. For this reason I have selected the three stars in Orion’s Belt and the six in his Sword, which have been long well-known groups, and I have added eighty other stars recently discovered in their vicinity, and I have preserved as exactly as possible the intervals between them. The well-known or old stars, for the sake of distinction, I have depicted of larger size, and I have outlined them with a double line; the others, invisible to the naked eye, I have marked smaller and with one line only. I have also preserved the differences of magnitude as much as I could.

For my first example, I had planned to draw the entire constellation of Orion. But the sheer number of stars and lack of time overwhelmed me, so I postponed the attempt until another occasion. More than five hundred new stars lie beside or among the previously known stars within one or two degrees. For this reason, I selected the three stars in Orion’s Belt and the six in his Sword. These groups have long been well known. I added eighty other stars recently discovered nearby, keeping the distances between them as accurate as possible. To distinguish the well-known stars, I drew them larger and outlined them with a double line. I drew the others, invisible to the naked eye, smaller and with only a single line. I also preserved the differences in brightness as closely as I could.

As a second example I have depicted the six stars of the constellation Taurus, called the Pleiades (I say six intentionally, since the seventh is scarcely ever visible), a group of stars which is enclosed in the heavens within very narrow precincts. Near these there lie more than forty others invisible to the naked eye, no one of which is much more than half a degree off any of the aforesaid six; of these I have noticed only thirty-six in my diagram. I have preserved their intervals, magnitudes, and the distinction between the old and the new stars, just as in the case of the constellation Orion.

As a second example, I have drawn the six stars in the constellation Taurus called the Pleiades. I say six intentionally, since the seventh is hardly ever visible. This group of stars occupies a very small area of the sky. Nearby are more than forty other stars invisible to the naked eye. None is much more than half a degree from any of those six stars. I have included only thirty-six of these other stars in my diagram. I have preserved their spacing, sizes, and the distinction between the previously known stars and the newly discovered stars. I did just the same for the constellation Orion.

The next object which I have observed is the essence or substance of the Milky Way. By the aid of a telescope any one may behold this in a manner which so distinctly appeals to the senses that all the disputes which have tormented philosophers through so many ages are exploded at once by the irrefragable evidence of our eyes, and we are freed from wordy disputes upon this subject, for the Galaxy is nothing else but a mass of innumerable stars planted together in clusters. Upon whatever part of it you direct the telescope straightway a vast crowd of stars presents itself to view; many of them are tolerably large and extremely bright, but the number of small ones is quite beyond determination.

The next thing I observed was the nature of the Milky Way. With a telescope, anyone can see its nature so clearly that the evidence of our eyes cannot be disputed. This evidence settles at once all the debates that have troubled philosophers for so many centuries. We are freed from wordy arguments on the subject, because the Galaxy is nothing but countless stars gathered in clusters. Wherever you point the telescope within the Milky Way, a vast crowd of stars immediately comes into view. Many are fairly large and extremely bright, but the number of small stars is far beyond counting.

And whereas that milky brightness, like the brightness of a white cloud, is not only to be seen in the Milky Way, but several spots of a similar colour shine faintly here and there in the heavens, if you turn the telescope upon any of them you will find a cluster of stars packed close together. Further—and you will be more surprised at this,—the stars which have been called by every one of the astronomers up to this day nebulous, are groups of small stars set thick together in a wonderful way, and although each one of them on account of its smallness, or its immense distance from us, escapes our sight, from the commingling of their rays there arises that brightness which has hitherto been believed to be the denser part of the heavens, able to reflect the rays of the stars or the Sun.

That milky glow, like the brightness of a white cloud, appears not only in the Milky Way. Several patches of a similar color also shine faintly here and there in the heavens. If you turn the telescope toward any of these patches, you will find a tightly packed cluster of stars. Furthermore, what follows will surprise you even more. The stars that all astronomers until now have called nebulous are groups of small stars packed remarkably close together. Although each star escapes our sight because of its small size or immense distance, their rays blend together. The combined rays produce the glow previously believed to come from a denser part of the heavens. That denser region was thought capable of reflecting the rays of the stars or the Sun.

I have observed some of these, and I wish to subjoin the star-clusters of two of these nebulæ. First, you have a diagram of the nebula called that of Orion’s Head, in which I have counted twenty-one stars.

I have observed some of these, and I wish to add illustrations of the star clusters in two of these nebulae. First, here is a diagram of the nebula called Orion’s Head. I have counted twenty-one stars in this nebula.

The second cluster contains the nebula called Præsepe, which is not one star only, but a mass of more than forty small stars. I have noticed thirty-six stars, besides the Aselli, arranged in the order of the accompanying diagram.

The second cluster contains the nebula called Præsepe. Præsepe is not just one star, but a group of more than forty small stars. I have observed thirty-six stars, besides the Aselli, arranged as shown in the accompanying diagram.

I have now finished my brief account of the observations which I have thus far made with regard to the Moon, the Fixed Stars, and the Galaxy. There remains the matter, which seems to me to deserve to be considered the most important in this work, namely, that I should disclose and publish to the world the occasion of discovering and observing four PLANETS, never seen from the very beginning of the world up to our own times, their positions, and the observations made during the last two months about their movements and their changes of magnitude; and I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day, owing to the restriction of my time. I give them warning however again, so that they may not approach such an inquiry to no purpose, that they will want a very accurate telescope, and such as I have described in the beginning of this account.

I have now finished my brief account of my observations so far of the Moon, the Fixed Stars, and the Galaxy. One matter remains, and it seems to me the most important in this work. I must tell the world how I came to discover and observe four PLANETS. These PLANETS had never been seen from the beginning of the world until our own time. I must also report their positions and my observations over the last two months of their movements and changing sizes. I call on all astronomers to study these PLANETS and determine how long they take to complete their orbits. I have not yet been able to determine these periods because my time has been limited. But I warn astronomers again that they will need a very accurate telescope, like the one described at the beginning of this account. Without such a telescope, their investigation will be fruitless.

On the 7th day of January in the present year, 1610, in the first¹⁴ hour of the following night, when I was viewing the constellations of the heavens through a telescope, the planet Jupiter presented itself to my view, and as I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, owing to want of power in my other telescope, namely, that three little stars, small but very bright, were near the planet; and although I believed them to belong to the number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic,¹⁵ and to be brighter than the rest of the stars, equal to them in magnitude. The position of them with reference to one another and to Jupiter was as follows (¹).

On January 7 of this year, 1610, in the first¹⁴ hour of the night, I was observing the constellations through a telescope. Jupiter came into view. I had made myself an excellent instrument, and I noticed something my other telescope had lacked the power to reveal. Three little stars, small but very bright, were near the planet. Although I believed these were fixed stars, their appearance puzzled me somewhat. They seemed to form an exact straight line, parallel to the ecliptic,¹⁵ and were brighter than other stars of equal size. Their positions relative to one another and to Jupiter were as follows (¹).

On the east side there were two stars, and a single one towards the west. The star which was furthest towards the east, and the western star, appeared rather larger than the third.

There were two stars to the east and one to the west. The easternmost star and the western star appeared somewhat larger than the third star.

I scarcely troubled at all about the distance between them and Jupiter, for, as I have already said, at first I believed them to be fixed stars; but when on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for there were three little stars all west of Jupiter, and nearer together than on the previous night, and they were separated from one another by equal intervals, as the accompanying illustration (²) shows.

I paid hardly any attention to their distance from Jupiter. As I have already said, I initially believed they were fixed stars. But on January 8th, led by some twist of fate, I looked again at the same part of the sky. I found a very different situation. There were three little stars, all west of Jupiter and closer together than on the previous night. The stars were equally spaced, as the accompanying illustration (²) shows.

At this point, although I had not turned my thoughts at all upon the approximation of the stars to one another, yet my surprise began to be excited, how Jupiter could one day be found to the east of all the aforesaid fixed stars when the day before it had been west of two of them; and forthwith I became afraid lest the planet might have moved differently from the calculation of astronomers, and so had passed those stars by its own proper motion. I therefore waited for the next night with the most intense longing, but I was disappointed of my hope, for the sky was covered with clouds in every direction.

At this point, I had not thought at all about the stars moving closer to one another. Yet I began to wonder with surprise how Jupiter could be east of all those fixed stars one day. The day before, Jupiter had been west of two of them. I immediately feared that the planet might have moved differently from astronomers’ calculations. Perhaps Jupiter had passed those stars through its own motion. So I waited for the next night with intense longing. But my hopes were disappointed, for clouds covered the sky in every direction.

But on January 10th the stars appeared in the following position with regard to Jupiter; there were two only, and both on the east side of Jupiter, the third, as I thought, being hidden by the planet (³). They were situated just as before, exactly in the same straight line with Jupiter, and along the Zodiac.

But on January 10th, the stars appeared in the following positions relative to Jupiter. Only two were visible, both east of Jupiter. I thought the third was hidden by the planet (³). Just as before, the stars were exactly in a straight line with Jupiter, along the Zodiac.

When I had seen these phenomena, as I knew that corresponding changes of position could not by any means belong to Jupiter, and as, moreover, I perceived that the stars which I saw had been always the same, for there were no others either in front or behind, within a great distance, along the Zodiac,—at length, changing from doubt into surprise, I discovered that the interchange of position which I saw belonged not to Jupiter, but to the stars to which my attention had been drawn, and I thought therefore that they ought to be observed henceforward with more attention and precision.

After seeing these phenomena, I knew that Jupiter could not possibly account for the corresponding changes in position. I also realized that the stars I saw had always been the same ones. There were no others for a great distance ahead or behind them along the Zodiac. So, at last, my doubt turned to surprise. I discovered that the changes in position came not from Jupiter, but from the stars that had caught my attention. I therefore thought I should observe these stars more carefully and precisely from then on.

Accordingly, on January 11th I saw an arrangement of the following kind (⁴), namely, only two stars to the east of Jupiter, the nearer of which was distant from Jupiter three times as far as from the star further to the east; and the star furthest to the east was nearly twice as large as the other one; whereas on the previous night they had appeared nearly of equal magnitude. I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun; which at length was established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions round Jupiter, observations of whose changes of position made with more exactness on succeeding nights the following account will supply. I have measured also the intervals between them with the telescope in the manner already explained. Besides this, I have given the times of observation, especially when several were made in the same night, for the revolutions of these planets are so swift that an observer may generally get differences of position every hour.

Accordingly, on January 11, I saw the following arrangement (⁴): only two stars lay east of Jupiter. The nearer star was three times as far from Jupiter as it was from the star farther east. The easternmost star was nearly twice as large as the other. Yet on the previous night, the two stars had appeared nearly equal in size. I therefore concluded without hesitation that three stars in the heavens move around Jupiter, just as Venus and Mercury circle the Sun. Many later observations eventually established this as clearly as daylight. These observations also established that not just three but four wandering celestial bodies revolve around Jupiter. The following account records their changing positions, observed more precisely on later nights. I also measured the distances between them with the telescope, using the method already explained. In addition, I recorded the observation times, especially when I made several observations in one night. These planets revolve so swiftly that an observer can generally detect changes in position every hour.

Jan. 12.—At the first hour of the next night I saw these heavenly bodies arranged in this manner (⁵). The satellite¹⁶ furthest to the east was greater than the satellite furthest to the west; but both were very conspicuous and bright; the distance of each one from Jupiter was two minutes. A third satellite, certainly not in view before, began to appear at the third hour; it nearly touched Jupiter on the east side, and was exceedingly small. They were all arranged in the same straight line, along the ecliptic.

Jan. 12. At the first hour of the next night, I saw these heavenly bodies arranged like this (⁵). The satellite¹⁶ farthest east was larger than the satellite farthest west. But both were very easy to see and bright. Each was two minutes of arc from Jupiter. A third satellite, certainly not visible before, began to appear at the third hour. This satellite nearly touched Jupiter on its east side and was extremely small. All the satellites lay in the same straight line along the ecliptic.

Jan. 13.—For the first time four satellites were in view in the following position with regard to Jupiter (⁶).

Jan. 13. For the first time, four satellites were visible in the following positions relative to Jupiter (⁶).

There were three to the west, and one to the east; they made a straight line nearly, but the middle satellite of those to the west deviated a little from the straight line towards the north. The satellite furthest to the east was at a distance of 2´ from Jupiter; there were intervals of 1´ only between Jupiter and the nearest satellite, and between the satellites themselves, west of Jupiter. All the satellites appeared of the same size, and though small they were very brilliant, and far outshone the fixed stars of the same magnitude.

Three satellites were to the west, and one was to the east. They formed an almost straight line. But the middle satellite on the western side lay slightly north of that line. The easternmost satellite was 2´ from Jupiter. To the west of Jupiter, the nearest satellite was only 1´ away, with only 1´ between neighboring satellites. All the satellites appeared the same size. Though small, they were very bright and far outshone the fixed stars of the same magnitude.

Jan. 14.—The weather was cloudy.

Jan. 14. The weather was cloudy.

Jan. 15.—At the third hour of the night the four satellites were in the state depicted in the next diagram ([^7]) with reference to Jupiter.

Jan. 15. At the third hour of the night, the four satellites were positioned as shown in the next diagram ([^7]) relative to Jupiter.

All were to the west, and arranged nearly in the same straight line; but the satellite which counted third from Jupiter was raised a little to the north. The nearest to Jupiter was the smallest of all; the rest appeared larger and in order of magnitude; the intervals between Jupiter and the three nearest satellites were all equal, and were of the magnitude of 2´ each; but the satellite furthest to the west was distant 4´ from the satellite nearest to it. They were very brilliant, and not at all twinkling, as they have always appeared both before and since. But at the seventh hour there were only three satellites, presenting with Jupiter an appearance of the following kind (⁸). They were, that is to say, in the same straight line to a hair; the nearest to Jupiter was very small, and distant from the planet 3´; the distance of the second from this one was 1´; and of the third from the second 4´ 30´´. But after another hour the two middle satellites were still nearer, for they were only 30´´, or less, apart.

All were west of Jupiter, arranged in nearly the same straight line. But the third satellite from Jupiter was slightly north of that line. The satellite nearest Jupiter was the smallest of all. The others appeared larger, increasing in size with their distance from Jupiter. The gaps from Jupiter to the nearest satellite and between the three nearest satellites were all equal, measuring 2´ each. But the westernmost satellite was 4´ from the satellite nearest it. The satellites were very bright and did not twinkle at all, just as they had always appeared before and since. But at the seventh hour, only three satellites were visible, forming the following arrangement with Jupiter (⁸). The satellites and Jupiter were in a perfectly straight line. The satellite nearest Jupiter was very small and 3´ from the planet. The second satellite was 1´ from the first, and the third was 4´ 30´´ from the second. But an hour later, the two middle satellites were even closer together, only 30´´ apart or less.

Jan. 16.—At the first hour of the night I saw three satellites arranged in this order (⁹). Jupiter was between two of them, which were at a distance of 0´ 40´´ from the planet on either side, and the third was west of Jupiter at a distance of 8´. The satellites near to Jupiter appeared brighter than the satellite further off, but not larger.

Jan. 16. In the first hour of the night, I saw three satellites arranged in this order (⁹). Jupiter was between two of them, each 0´ 40´´ from the planet on opposite sides. The third satellite was 8´ west of Jupiter. The satellites closer to Jupiter looked brighter than the more distant satellite, but not larger.

Jan. 17, after sunset 0 hours 30 minutes, the configuration was of this kind (¹⁰). There was one satellite only to the east, at a distance of 3´ from Jupiter; to the west likewise there was only one satellite, distant 11´ from Jupiter. The satellite on the east appeared twice as large as the satellite to the west; and there were no more than these two. But four hours after, that is, nearly at the fifth hour, a third satellite began to emerge on the east side, which, before its appearance, as I think, had been joined with the former of the two other satellites, and the position was of this kind (¹¹). The middle satellite was very near indeed to the satellite on the east, and was only 20´´ from it; and was a little towards the south of the straight line drawn through the two extreme satellites and Jupiter.

Jan. 17, 30 minutes after sunset, the arrangement looked like this (¹⁰). There was only one satellite to the east, 3´ from Jupiter. Likewise, there was only one satellite to the west, 11´ from Jupiter. The eastern satellite appeared twice as large as the western satellite. There were no more than these two. But four hours later, nearly at the fifth hour, a third satellite began to emerge on the east side. Before becoming visible, I think, the third satellite had been joined with the eastern satellite. The arrangement then looked like this (¹¹). The middle satellite was very close to the eastern satellite, only 20´´ away. The middle satellite was slightly south of the straight line through the two outermost satellites and Jupiter.

Jan. 18, at 0 h. 20 m. after sunset, the appearance was such as this (¹²). The satellite to the east was larger than the western one, and was at a distance from Jupiter of 8´, the western one being at a distance of 10´.

Jan. 18, 20 minutes after sunset, the arrangement looked like this (¹²). The satellite east of Jupiter was larger than the western one and was 8´ from Jupiter. The western satellite was 10´ from Jupiter.

Jan. 19.—At the second hour of the night the relative position of the satellites was such as this (¹³); that is, there were three satellites exactly in a straight line with Jupiter, one to the east, at a distance of 6´ from Jupiter; between Jupiter and the first satellite to the west in order, there was an interval of 5´; this satellite was 4´ off the other one more to the west. At that time I was doubtful whether or no there was a satellite between the satellite to the east and Jupiter, but so very close to Jupiter as almost to touch the planet; but at the fifth hour I saw this satellite distinctly, by that time occupying exactly the middle position between Jupiter and the eastern satellite, so that the configuration was thus (¹⁴). Moreover, the satellite which had just come into view was very small; yet at the sixth hour it was nearly as large as the rest.

Jan. 19. At the second hour of the night, the satellites were arranged like this (¹³). Three satellites lay exactly in a straight line with Jupiter. One was to the east, 6´ from Jupiter. The nearest satellite to the west was 5´ from Jupiter and 4´ from the satellite farther west. At that time, I was unsure whether another satellite lay between the eastern satellite and Jupiter. If present, it was so close to Jupiter that it almost touched the planet. But at the fifth hour, I saw this satellite clearly. By then, it was exactly halfway between Jupiter and the eastern satellite, so the arrangement looked like this (¹⁴). Moreover, the newly visible satellite was very small. Yet at the sixth hour, it was nearly as large as the others.

Jan. 20: 1 h. 15 m.—A similar arrangement was seen (¹⁵). There were three satellites, so small as scarcely to be distinguishable; their distances from Jupiter, and from one another, were not more than 1´. I was doubtful whether on the western side there were two satellites or three. About the sixth hour they were grouped in this way (¹⁶). The eastern satellite was twice as far away from Jupiter as before, that is 2´; on the western side, the satellite in the middle was distant from Jupiter 0´ 40´´, and from the satellite still further to the west 0´ 20´´; at length, at the seventh hour, three satellites were seen on the western side (¹⁷). The satellite nearest to Jupiter was distant from the planet 0´ 20´´; between this one and the satellite furthest to the west there was an interval of 40´´, but between these another satellite was in view slightly southward of them, and not more than 10´´ off the most westerly satellite.

Jan. 20: 1 h. 15 m. A similar arrangement was seen (¹⁵). There were three satellites, so small they were barely distinguishable. Their distances from Jupiter and from one another were no more than 1´. I was unsure whether there were two satellites or three on the western side. Around the sixth hour, they were grouped like this (¹⁶). The eastern satellite was twice as far from Jupiter as before, at 2´. On the western side, the middle satellite was 0´ 40´´ from Jupiter. It was 0´ 20´´ from the satellite farther west. Finally, at the seventh hour, three satellites were seen on the western side (¹⁷). The satellite nearest Jupiter was 0´ 20´´ from the planet. The gap between this satellite and the farthest west was 40´´. But between these two, another satellite was visible slightly to their south. It was no more than 10´´ from the westernmost satellite.

Jan. 21: 0 h. 30 m.—There were three satellites on the east side; the satellites and Jupiter were at equal distances apart (¹⁸). The intervals were by estimation 50´´ each. There was also one satellite on the west, distant 4´ from Jupiter. The satellite on the east side nearest to Jupiter was the least of all.

Jan. 21: 0 h. 30 m. There were three satellites on the east side. The satellites and Jupiter were equally spaced (¹⁸). Each gap was estimated at 50´´. There was also one satellite on the west side, 4´ from Jupiter. The satellite on the east side closest to Jupiter was the smallest of all.

Jan. 22: 2 h.—The grouping of the satellites was similar (¹⁹). There was an interval of 5´ from the satellite on the east to Jupiter; from Jupiter to the satellite furthest to the west 7´. The two interior satellites on the western side were 0´ 40´´ apart, and the satellite nearer to Jupiter was 1´ from the planet. The inner satellites were smaller than the outer ones, but they were situated all in the same straight line, along the ecliptic, except that the middle of the three western satellites was slightly to the south of it, but at the sixth hour of the night they appeared in this position (²⁰). The satellite on the east was very small, at a distance from Jupiter of 5´ as before; but the three satellites on the west were separated by equal distances from Jupiter and from each other; and the intervals were nearly 1´ 20´´ each. The satellite nearest Jupiter appeared smaller than the other two on the same side, but they all appeared arranged exactly in the same straight line.

Jan. 22, 2 h.: The satellites were grouped similarly (¹⁹). The eastern satellite was 5´ from Jupiter. The westernmost satellite was 7´ from Jupiter. The two inner satellites on the western side were 0´ 40´´ apart. The satellite closer to Jupiter was 1´ from the planet. The inner satellites were smaller than the outer ones. All lay on the same straight line along the ecliptic, except the middle of the three western satellites. That satellite was slightly south of the line. But at the sixth hour of the night, the satellites appeared in this position (²⁰). The eastern satellite was very small and remained 5´ from Jupiter. But the three western satellites were equally spaced from Jupiter and from one another. Each gap was nearly 1´ 20´´. The satellite nearest Jupiter appeared smaller than the other two on the western side. But all the satellites appeared to lie exactly on the same straight line.

Jan. 23, at 0 h. 40 m. after sunset, the grouping of the satellites was nearly after this fashion (²¹). There were three satellites with Jupiter in a straight line along the ecliptic, as they always have been; two were on the east of the planet, one on the west; the satellite furthest to the east was 7´ from the next one, and this satellite 2´ 40´´ from Jupiter; Jupiter was 3´ 20´´ from the satellite on the west; and they were all of nearly the same size. But at the fifth hour the two satellites which had been previously near Jupiter were no longer visible, being, as I suppose, hidden behind Jupiter, and the appearance presented was such as this (²²).

Jan. 23, 40 minutes after sunset, the satellites were arranged roughly like this (²¹). Three satellites formed a straight line with Jupiter along the ecliptic, as they always have. Two were east of the planet, and one was west. The easternmost satellite was 7´ from the next satellite, which was 2´ 40´´ from Jupiter. Jupiter was 3´ 20´´ from the satellite to the west. All the satellites were nearly the same size. But at the fifth hour, the two satellites previously near Jupiter were no longer visible. I suppose they were hidden behind Jupiter. The view then looked like this (²²).

Jan. 24.—Three satellites, all on the east side, were visible, and nearly, but not quite, in the same straight line with Jupiter, for the satellite in the middle was slightly to the south of it (²³). The satellite nearest to Jupiter was 2´ distant from the planet; the next in order 0´ 30´´ from this satellite, and the third was 9´ further off still; they were all very bright. But at the sixth hour two satellites only presented themselves to view in this position, namely in the same straight line with Jupiter exactly, and the distance of the nearest to the planet was lengthened to 3´; the next was 2´ further off, and unless I am mistaken, the two satellites previously observed in the middle had come together, and appeared as one.

Jan. 24. Three satellites were visible, all on the east side and almost, but not quite, aligned with Jupiter. The middle satellite was slightly south of this line (²³). The satellite nearest Jupiter was 2´ from the planet. The next was 0´ 30´´ from the nearest satellite, and the third was another 9´ farther away. All three were very bright. But at the sixth hour, only two satellites were visible, exactly aligned with Jupiter. The distance from Jupiter to the nearest satellite had increased to 3´. The next satellite was another 2´ farther away. Unless I am mistaken, the two satellites previously observed in the middle had come together and appeared as one.

Jan. 25, at 1 h. 40 m., the satellites were grouped thus (²⁴). There were only two satellites on the east side, and these were rather large. The satellite furthest to the east was 5´ from the satellite in the middle, and it was 6´ from Jupiter.

Jan. 25, at 1 h. 40 m., the satellites were arranged as shown (²⁴). Only two satellites were on the east side, and both were rather large. The easternmost satellite was 5´ from the middle satellite and 6´ from Jupiter.

Jan. 26, at 0 h. 40 m., the relative positions of the satellites were thus (²⁵). Three satellites were in view, of which two were east and the third west of Jupiter; this one was distant 3´ from the planet. On the east side the satellite in the middle was at a distance of 5´ 20´´; the further satellite was 6´ beyond; they were arranged in a straight line, and were of the same size. At the fifth hour the arrangement was nearly the same, with this difference only, that the fourth satellite was emerging on the east side near Jupiter. It was smaller than the rest, and was then at a distance of 0´ 30´´ from Jupiter; but was raised a little above the straight line towards the north, as the accompanying figure shows (²⁶).

Jan. 26, at 0 h. 40 m., the satellites’ relative positions were as follows (²⁵). Three satellites were visible, two east of Jupiter and one west. The western satellite was 3´ from the planet. On the east side, the middle satellite was 5´ 20´´ from Jupiter. The farther satellite was 6´ beyond the middle satellite. The three satellites lay in a straight line and were the same size. At the fifth hour, the arrangement was nearly the same, with only one difference. The fourth satellite was emerging close to Jupiter on the east side. This satellite was smaller than the others and was then 0´ 30´´ from Jupiter. But it lay slightly north of the straight line, as the accompanying figure shows (²⁶).

Jan. 27, 1 h. after sunset, a single satellite only was in view, and that on the east side of Jupiter in this position (²⁷). It was very small, and at a distance of 7´ from Jupiter.

January 27, one hour after sunset, only one satellite was visible. It was east of Jupiter, in this position (²⁷). The satellite was very small and 7 arcminutes from Jupiter.

Jan. 28 and 29.—Owing to the intervention of clouds, I could make no observation.

Jan. 28 and 29. Clouds prevented me from making any observations.

Jan. 30.—At the first hour of the night the satellites were in view, arranged in the following way (²⁸). There was one satellite on the east side, at a distance of 2´ 30´´ from Jupiter; and there were two satellites on the west, of which the one nearer to Jupiter was 3´ off the planet, and the other satellite 1´ further. The places of the outer satellites and Jupiter were in the same straight line; but the satellite in the middle was a little above it to the north. The satellite furthest to the west was smaller than the rest.

Jan. 30. In the first hour of the night, the satellites were visible, arranged as follows (²⁸). One satellite was east of Jupiter, at a distance of 2´ 30´´. Two satellites were to the west. The one closer to Jupiter was 3´ from the planet, and the other was 1´ farther away. The outer satellites and Jupiter lay on the same straight line. But the middle satellite was slightly above that line, to the north. The satellite farthest west was smaller than the others.

On the last day of the month, at the second hour, two satellites on the east side were visible, and one on the west (²⁹). Of the satellites east of the planet, the one in the middle was 2´ 20´´ distant from Jupiter; and the satellite further to the east was 0´ 30´´ from the middle satellite; the satellite on the west was at a distance of 10´ from Jupiter. They were in the same straight line nearly, and would have been exactly so, only the satellite on the east nearest to Jupiter was raised a little towards the north. At the fourth hour, the two satellites on the east were still nearer together, for they were only 20´´ apart (³⁰). The western satellite appeared rather small in these two observations.

On the last day of the month, at the second hour, two satellites were visible east of Jupiter. One was visible to the west (²⁹). Of the satellites east of Jupiter, the middle one was 2´ 20´´ from the planet. The satellite farther east was 0´ 30´´ from the middle satellite. The western satellite was 10´ from Jupiter. The satellites lay almost in a straight line. They would have lined up exactly, except that the eastern satellite nearest Jupiter was slightly north of the line. At the fourth hour, the two eastern satellites were even closer together, only 20´´ apart (³⁰). The western satellite appeared rather small in both observations.

Feb. 1.—At the second hour of the night the arrangement was similar ([^31]). The satellite furthest to the east was at a distance of 6´ from Jupiter, and the western satellite 8´. On the east side there was a very small satellite, at a distance of 20´´ from Jupiter. They made a perfectly straight line.

Feb. 1. At the second hour of the night, the arrangement was similar ([^31]). The satellite farthest east was 6´ from Jupiter, and the western satellite was 8´ away. A very small satellite was on the east side, 20´´ from Jupiter. The satellites formed a perfectly straight line.

Feb. 2.—The satellites were seen arranged thus ([^32]). There was one only on the east, at a distance of 6´ from Jupiter. Jupiter was 4´ from the nearest satellite on the west; between this satellite and the satellite further to the west there was an interval of 8´; they were in the same straight line exactly, and were nearly of the same magnitude. But at the seventh hour four satellites were there—two on each side of Jupiter ([^33]). Of these satellites, the most easterly was at a distance of 4´ from the next; this satellite was 1´ 40´´ from Jupiter; Jupiter was 6´ from the nearest satellite on the west, and this one from the satellite further to the west, 8´; and they were all alike in the same straight line, drawn in the direction of the Zodiac.

Feb. 2. The satellites appeared in this arrangement ([^32]). Only one was east of Jupiter, at a distance of 6´. Jupiter was 4´ from the nearest satellite to the west. The two western satellites were 8´ apart. The satellites lay exactly on the same straight line and were nearly the same size. But at the seventh hour, four satellites were visible, two on each side of Jupiter ([^33]). The easternmost satellite was 4´ from the other eastern satellite. The latter was 1´ 40´´ from Jupiter. Jupiter was 6´ from the nearest western satellite. The two western satellites were 8´ apart. All the satellites lay on the same straight line, running in the direction of the Zodiac.

Feb. 3: 7 h.—The satellites were arranged in the following way ([^34]):—The satellite on the east was at a distance of 1´ 30´´ from Jupiter; the nearest satellite on the west, 2´, and there was a long distance, 10´, from this satellite to the satellite further to the west. They were exactly in the same straight line, and of equal magnitude.

Feb. 3, 7 h.: The satellites were arranged as follows ([^34]): The eastern satellite was 1´ 30´´ from Jupiter. The nearest western satellite was 2´ from Jupiter. A large gap of 10´ separated this satellite from the satellite farther west. All the satellites lay exactly on the same straight line and were equally bright.

Feb. 4: 2 h.—Four satellites attended Jupiter, two on the east and two on the west, arranged in one perfectly straight line, as in the adjoining figure ([^35]). The satellite furthest to the east was at a distance of 3´ from the next satellite. This one was 0´ 40´´ from Jupiter; Jupiter 4´ from the nearest satellite on the west; and this one from the satellite further to the west 6´. In magnitude they were nearly equal; the satellite nearest to Jupiter was rather smaller in appearance than the rest. But at the seventh hour ([^36]) the eastern satellites were at a distance of only 0´ 30´´ apart. Jupiter was 2´ from the nearest satellite on the east; and from the satellite on the west, next in order, 4´; this one was distant 3´ from the satellite further to the west. They were all equal in magnitude, and in a straight line, drawn in the direction of the ecliptic.

Feb. 4: 2 h. Four satellites accompanied Jupiter, two to the east and two to the west. They formed a perfectly straight line, as in the accompanying figure ([^35]). The easternmost satellite was 3´ from the next satellite. That satellite was 0´ 40´´ from Jupiter. Jupiter was 4´ from the nearest satellite to the west. That western satellite was 6´ from the satellite farther west. The satellites appeared nearly equal in size. The satellite nearest Jupiter appeared slightly smaller than the others. But at the seventh hour ([^36]), the eastern satellites were only 0´ 30´´ apart. Jupiter was 2´ from the nearest satellite to the east and 4´ from the next satellite, to the west. That western satellite was 3´ from the satellite farther west. All four satellites appeared equal in size and formed a straight line in the direction of the ecliptic.

Feb. 5.—The sky was cloudy.

February 5. The sky was cloudy.

Feb. 6.—Two satellites only appeared, with Jupiter between them, as is seen in the accompanying figure ([^37]). The satellite on the east was 2´ from Jupiter, and that on the west 3´. They were in the same straight line with Jupiter, and were equal in magnitude.

Feb. 6. Only two satellites were visible, with Jupiter between them, as shown in the accompanying figure ([^37]). The eastern satellite was 2´ from Jupiter, and the western satellite was 3´ away. Both satellites lay on the same straight line as Jupiter and appeared equal in size.

Feb. 7.—There were two satellites by the side of Jupiter, and both of them on the east of the planet, arranged in this manner ([^38]). The intervals between the satellites and Jupiter were equal, and of 1´ each; and a straight line would go through them and the centre of Jupiter.

Feb. 7. Two satellites were beside Jupiter, both east of the planet, arranged as shown ([^38]). The gaps between Jupiter and the satellites were equal, each measuring 1´. A straight line would pass through both satellites and Jupiter’s center.

Feb. 8: 1 h.—Three satellites were there, all on the east side of Jupiter, as in the diagram ([^39]). The nearest to Jupiter, a rather small one, was distant from the planet 1´ 20´´; the middle one was 4´ from this satellite, and was rather large; the satellite furthest to the east, a very small one, was at a distance of 0´ 20´´ from this satellite. It was doubtful whether there was one satellite near to Jupiter or two, for sometimes it seemed that there was another satellite by its side towards the east, wonderfully small, and only 10´´ from it. They were all situated at points in a straight line drawn in the direction of the Zodiac. At the third hour the satellite nearest to Jupiter was almost touching the planet, for it was only distant 10´´ from it; but the others had become further off, for the middle one was 6´ from Jupiter. At length, at the fourth hour, the satellite which was previously the nearest to Jupiter joined with the planet and disappeared.

Feb. 8: 1 h. Three satellites were visible, all east of Jupiter, as shown in the diagram ([^39]). The nearest satellite was rather small and was 1´ 20´´ from Jupiter. The middle satellite was 4´ from the nearest satellite and was rather large. The easternmost satellite was very small and was 0´ 20´´ from the middle satellite. It was unclear whether there was one satellite near Jupiter or two. At times, another remarkably small satellite seemed to appear just east of the nearest satellite, only 10´´ away. All the satellites lay on a straight line running in the direction of the Zodiac. At the third hour, the satellite nearest Jupiter was almost touching the planet, only 10´´ away. But the other satellites had moved farther away, with the middle satellite now 6´ from Jupiter. Finally, at the fourth hour, the satellite previously nearest Jupiter merged with the planet and disappeared.

Feb. 9: 0 h. 30 m.—There were two satellites on the east side of Jupiter, and one on the west, in an arrangement such as this ([^40]). The satellite furthest to the east, which was a rather small one, was distant 4´ from the next satellite; the satellite in the middle was larger, and at a distance of 7´ from Jupiter. Jupiter was distant 4´ from the western satellite, which was a small one.

Feb. 9: 0 h. 30 m. Two satellites were east of Jupiter, and one was west, arranged like this ([^40]). The easternmost satellite was rather small and was 4´ from the next satellite. The middle satellite was larger and was 7´ from Jupiter. Jupiter was 4´ from the small western satellite.

Feb. 10: 1 h. 30 m.—A pair of satellites, very small, and both on the east of the planet, were visible, in the following position ([^41]). The further satellite was distant from Jupiter 10´, the nearer 0´ 20´´, and they were in the same straight line; but at the fourth hour the satellite nearest to Jupiter no longer appeared, and the other seemed so diminished that it could scarcely be kept in sight, although the atmosphere was quite clear, and the satellite was further from Jupiter than before, since its distance was now 12´.

Feb. 10: 1 h. 30 m. Two very small satellites were visible, both east of the planet, in the following position ([^41]). The farther satellite was 10´ from Jupiter, and the nearer one was 0´ 20´´ away. Both lay on the same straight line. But by the fourth hour, the satellite nearest Jupiter was no longer visible. The other seemed so much smaller that it could barely be kept in view, although the atmosphere was quite clear. The satellite was also farther from Jupiter than before, since its distance was now 12´.

Feb. 11: 1 h.—There were two satellites on the east, and one on the west ([^42]). The western satellite was at a distance of 4´ from Jupiter. The satellite on the east, nearest to the planet, was likewise 4´ from Jupiter; but the satellite further to the east was at a distance from this one of 8´; they were fairly clear to view, and in the same straight line; but at the third hour the fourth satellite was visible near to Jupiter on the east, less in magnitude than the rest, separated from Jupiter by a distance of 0´ 30´´, and slightly to the north out of the straight line drawn through the rest ([^43]). They were all very bright and extremely distinct, but at 5 h. 30 m. the satellite on the east nearest to Jupiter had moved further away from the planet, and was occupying a position midway between the planet and the neighbouring satellite further to the east. They were all in the same straight line exactly, and of the same magnitude, as may be seen in the accompanying diagram ([^44]).

Feb. 11, 1 h.: There were two satellites to the east and one to the west ([^42]). The western satellite was 4´ from Jupiter. The eastern satellite nearest the planet was also 4´ from Jupiter. But the satellite farther east was 8´ from the nearer eastern satellite. The satellites were fairly clear to see and lay in the same straight line. But at the third hour, the fourth satellite became visible close to Jupiter on the east. It appeared smaller than the others and was 0´ 30´´ from Jupiter. It lay slightly north of the straight line through the other satellites ([^43]). All the satellites were very bright and extremely distinct. But at 5 h. 30 m., the eastern satellite nearest Jupiter had moved farther from the planet. It was now midway between Jupiter and the neighboring satellite farther east. All the satellites lay in exactly the same straight line and appeared the same size, as the accompanying diagram shows ([^44]).

Feb. 12: 0 h. 40 m.—A pair of satellites on the east, a pair likewise on the west, were near the planet ([^45]). The satellite on the east furthest removed from Jupiter was at a distance of 10´, and the further of the satellites on the west was 8´ off. They were both fairly distinct. The other two were very near to Jupiter, and very small, especially the satellite to the east, which was at a distance of 0´ 40´´ from Jupiter. The distance of the western satellite was 1´. But at the fourth hour the satellite which was nearest to Jupiter on the east was visible no longer.

Feb. 12: 0 h. 40 m.: Two satellites to the east and two to the west were near the planet ([^45]). The eastern satellite farthest from Jupiter was 10´ away, and the farther western satellite was 8´ away. Both were fairly distinct. The other two satellites were very close to Jupiter and very small, especially the eastern satellite. The eastern satellite was 0´ 40´´ from Jupiter. The western satellite was 1´ away. But by the fourth hour, the eastern satellite nearest Jupiter was no longer visible.

Feb. 13: 0 h. 30 m.—Two satellites were visible in the east, two also in the west ([^46]). The satellite on the east near Jupiter was fairly distinct; its distance from the planet was 2´. The satellite further to the east was less noticeable; it was distant 4´ from the other. Of the satellites on the west, the one furthest from Jupiter, which was very distinct, was parted from the planet 4´. Between this satellite and Jupiter intervened a small satellite close to the most westerly satellite, being not more than 0´ 3´´ off. They were all in the same straight line, corresponding exactly to the direction of the ecliptic.

Feb. 13: 0 h. 30 m. Two satellites were visible to the east, and two to the west ([^46]). The eastern satellite nearer Jupiter was fairly distinct, at a distance of 2´ from the planet. The satellite farther east was less noticeable, at a distance of 4´ from the nearer satellite. Of the western satellites, the one farthest from Jupiter was very distinct and 4´ from the planet. A small satellite lay between this outermost western satellite and Jupiter. The small satellite was close to the outermost western satellite, no more than 0´ 3´´ away. All four satellites lay in the same straight line, aligned exactly with the ecliptic.

Feb. 15 (for on the 14th the sky was covered with clouds), at the first hour, the position of the satellites was thus ([^47]); that is, there were three satellites on the east, but none were visible on the west. The satellite on the east nearest to Jupiter was at a distance of 0´ 50´´ from the planet; the next in order was 0´ 20´´ from this satellite, and the furthest to the east was 2´ from the second satellite, and it was larger than the others, for those nearer to Jupiter were very small. But about the fifth hour only one of the satellites which had been near to Jupiter was to be seen; its distance from Jupiter was 0´ 30´´. The distance of the satellite furthest to the east from Jupiter had increased, for it was then 4´ ([^48]). But at the sixth hour, besides the two situated as just described on the east, one satellite was visible towards the west, very small, at a distance of 2´ from Jupiter ([^49]).

Feb. 15 (the sky was covered with clouds on the 14th). At the first hour, the satellites were positioned as follows ([^47]). Three satellites were east of Jupiter, but none were visible to the west. The eastern satellite nearest Jupiter was 0´ 50´´ from the planet. The next satellite was 0´ 20´´ from the nearest one. The farthest east was 2´ from the second satellite. The farthest satellite was larger than the others, for the satellites nearer Jupiter were very small. But around the fifth hour, only one of the satellites that had been near Jupiter was visible. That satellite was 0´ 30´´ from Jupiter. The distance between Jupiter and the satellite farthest east had increased to 4´ ([^48]). But at the sixth hour, another satellite was visible to the west, besides the two just described to the east. The western satellite was very small and 2´ from Jupiter ([^49]).

Feb. 16: 6 h.—Their places were arranged as follows ([^50]); that is, the satellite on the east was 7´ from Jupiter, Jupiter 5´ from the next satellite on the west, and this 3´ from the remaining satellite still further to the west. They were all of the same magnitude nearly, rather bright, and in the same straight line, corresponding accurately to the direction of the Zodiac.

Feb. 16: 6 h. Their positions were as follows ([^50]). The satellite to the east was 7´ from Jupiter. Jupiter was 5´ from the next satellite to the west. That satellite was 3´ from the remaining satellite, still farther west. All were nearly equal in brightness and rather bright. All lay in the same straight line, aligned precisely with the direction of the Zodiac.

Feb. 17: 1 h.—Two satellites were in view, one on the east, distant 3´ from Jupiter; the other on the west, distant 10´ ([^51]). The latter was somewhat less than the satellite on the east; but at the sixth hour the eastern satellite was nearer to Jupiter, being at a distance of 0´ 50´´, and the western satellite was further off, namely 12´. At both observations they were in the same straight line with Jupiter, and were both rather small, especially the eastern satellite in the second observation.

Feb. 17, 1 h.: Two satellites were visible. One was east of Jupiter, 3´ away. The other was west of Jupiter, 10´ away ([^51]). The western satellite was somewhat smaller than the eastern one. But by the sixth hour, the eastern satellite was closer to Jupiter, at a distance of 0´ 50´´. The western satellite was farther away, at 12´. In both observations, the two satellites formed a straight line with Jupiter. Both satellites were rather small, especially the eastern satellite in the second observation.

Feb. 18: 1 h.—Three satellites were in view, of which two were on the west and one on the east; the distance of the eastern satellite from Jupiter was 3´, and of the nearest satellite on the west 2´; the remaining satellite, still further to the west, was 8´ from the middle satellite ([^52]). They were all in the same straight line exactly, and of about the same magnitude. But at the second hour the satellites nearest to the planet were at equal distances from Jupiter, for the western satellite was now also 3´ from the planet. But at the sixth hour the fourth satellite was visible between the satellite on the east and Jupiter, in the following configuration ([^53]). The satellite furthest to the east was at a distance of 3´ from the next in order; this one was at a distance of 1´ 50´´ from Jupiter; Jupiter was at a distance of 3´ from the next satellite on the west, and this 7´ from the satellite still further to the west. These were nearly equal in magnitude, only the satellite on the east nearest to Jupiter was a little smaller than the rest, and they were all in the same straight line parallel to the ecliptic.

Feb. 18: 1 h.: Three satellites were visible, two to the west and one to the east. The eastern satellite was 3′ from Jupiter, and the nearest western satellite was 2′ from Jupiter. The remaining satellite, farther west, was 8′ from the middle satellite ([^52]). All three lay exactly on the same straight line and were about equal in brightness. But at the second hour, the satellites nearest Jupiter were equally distant from the planet. The western satellite was now also 3′ from Jupiter. But at the sixth hour, the fourth satellite was visible between the eastern satellite and Jupiter, in this arrangement ([^53]). The easternmost satellite was 3′ from the next satellite, which was 1′ 50″ from Jupiter. Jupiter was 3′ from the next satellite to the west. That western satellite was 7′ from the satellite farther west. The satellites were nearly equal in brightness, though the eastern satellite nearest Jupiter was a little fainter than the others. All lay on the same straight line, parallel to the ecliptic.

Feb. 19: 0 h. 40 m.—Two satellites only were in view, west of Jupiter, rather large, and arranged exactly in the same straight line with Jupiter, in the direction of the ecliptic ([^54]). The nearer satellite was at a distance of 7´ from Jupiter and of 6´ from the satellite further to the west.

Feb. 19: 0 h. 40 m. Only two satellites were visible, both west of Jupiter and rather large. Both lay exactly on a straight line with Jupiter, in the direction of the ecliptic ([^54]). The nearer satellite was 7´ from Jupiter and 6´ from the satellite farther west.

Feb. 20.—The sky was cloudy.

Feb. 20. The sky was cloudy.

Feb. 21: 1 h. 30 m.—Three satellites, rather small, were in view, placed thus ([^55]). The satellite to the east was 2´ from Jupiter; Jupiter was 3´ from the next, which was on the west; and this one was 7´ from the satellite further to the west. They were exactly in the same straight line parallel to the ecliptic.

Feb. 21: 1 h. 30 m. Three rather small satellites were visible, arranged like this ([^55]). The satellite to the east was 2´ from Jupiter. Jupiter was 3´ from the next satellite, to the west. That satellite was 7´ from the satellite farther west. All three satellites lay exactly on the same straight line, parallel to the ecliptic.

Feb. 25: 1 h. 30 m. (for on the three previous nights the sky was overcast).—Three satellites appeared, two on the east, which were at a distance of 4´ apart, the same as the distance of the nearer satellite from Jupiter; on the west there was one satellite at a distance of 2´ from Jupiter. They were exactly in the same straight line in the direction of the ecliptic ([^56]).

Feb. 25: 1 h. 30 m. (The sky had been overcast on the three previous nights.) Three satellites appeared, two to the east. The two eastern satellites were 4´ apart, the same distance as the nearer satellite was from Jupiter. One satellite was to the west, 2´ from Jupiter. All three satellites lay exactly on the same straight line, in the direction of the ecliptic ([^56]).

Feb. 26: 0 h. 30 m.—A pair of satellites only were present, one on the east, distant 10´ from Jupiter; the other was on the west, at a distance of 6´ ([^57]). The eastern satellite was slightly smaller than the western. At the fifth hour three satellites were visible; for, besides the two already noticed, a third satellite was in view, on the west, near Jupiter, very small, which previously had been hidden behind Jupiter, and it was at a distance of 1´ from the planet ([^58]).

Feb. 26: 0 h. 30 m. Only two satellites were visible. One was east of Jupiter, 10´ away. The other was west of Jupiter, 6´ away ([^57]). The eastern satellite was slightly smaller than the western one. At the fifth hour, three satellites were visible, because a third had appeared besides the two already noted. The third satellite was very small and lay west of Jupiter, close to the planet. This satellite had previously been hidden behind Jupiter and was now 1´ from the planet ([^58]).

But the satellite on the east was seen to be further off than before, being at a distance of 11´ from Jupiter. On this night, for the first time, I determined to observe the motion of Jupiter and the adjacent planets (his satellites) along the zodiac, by reference to some fixed star; for there was a fixed star in view, eastwards of Jupiter, at a distance of 11´ from the satellite on the east, and a little to the south, in the following manner ([^59]).

But the satellite to the east appeared farther away than before, at a distance of 11´ from Jupiter. That night, for the first time, I decided to track Jupiter and the nearby planets (its satellites) along the zodiac. I would use a fixed star as a reference. For a fixed star was visible east of Jupiter, 11´ from the eastern satellite and a little to the south. The arrangement was as follows ([^59]).

Feb. 27: 1 h. 4 m.—The satellites appeared in the following configuration. The satellite furthest to the east was at a distance of 10´ from Jupiter; the next in order was near Jupiter, being at a distance of 0´ 30´´ from the planet. The next satellite was on the western side, at a distance of 2´ 30´´ from Jupiter; and the satellite further to the west was at a distance of 1´ from this. The two satellites near to Jupiter appeared small, especially the satellite on the east; but the satellites furthest off were very bright, particularly that on the west, and they made a straight line in the direction of the ecliptic exactly. The motion of the planets towards the east was plainly seen by reference to the aforesaid fixed star, for Jupiter and his attendant satellites were nearer to it, as may be seen in the accompanying figure ([^60]). At the fifth hour the satellite on the east, near to Jupiter, was 1´ from the planet.

Feb. 27: 1 h. 4 m. The satellites appeared in the following arrangement. The satellite farthest east was 10´ from Jupiter. The next was close to Jupiter, at a distance of 0´ 30´´. The next satellite was on the western side, 2´ 30´´ from Jupiter. The satellite farther west was 1´ from that satellite. The two satellites close to Jupiter appeared small, especially the one to the east. But the farthest satellites were very bright, particularly the western one. All four formed a perfectly straight line along the ecliptic. The planets’ eastward motion was clearly visible relative to the fixed star mentioned earlier. Jupiter and its accompanying satellites were closer to the star, as shown in the accompanying figure ([^60]). At the fifth hour, the eastern satellite close to Jupiter was 1´ from the planet.

Feb. 28: 1 h.—Only two satellites were visible, one on the east, at a distance of 9´ from Jupiter, and another on the west, at a distance of 2´; they were both rather bright, and in the same straight line with Jupiter, and a straight line drawn from the fixed star perpendicular to this straight line fell upon the satellite on the east, as in the figure ([^61]). At the fifth hour a third satellite was seen at a distance of 2´ from Jupiter, on the east, in the position shown in the figure ([^62]).

Feb. 28, 1 h.: Only two satellites were visible. One was east of Jupiter, at a distance of 9´, and the other was west, at a distance of 2´. Both were fairly bright and lay in a straight line with Jupiter. A line drawn from the fixed star, perpendicular to the line through Jupiter and the satellites, reached the eastern satellite. This is shown in the figure ([^61]). At the fifth hour, a third satellite appeared 2´ east of Jupiter, in the position shown in the figure ([^62]).

March 1: 0 h. 40 m.—Four satellites, all on the east of the planet, were seen; the satellite nearest to Jupiter was 2´ from the planet; the next 1´ from this; the third was 0´ 20´´ from the second, and was brighter than the others; and the satellite still further to the east was at a distance of 4´ from it, and was smaller than the others ([^63]). They made a straight line very nearly, only the satellite third from Jupiter was slightly above the line. The fixed star formed with Jupiter and the most easterly satellite an equilateral triangle, as in the figure.

March 1: 0 h. 40 m. Four satellites were visible, all east of the planet. The satellite nearest to Jupiter was 2´ from the planet. The next was 1´ from the nearest satellite. The third was 0´ 20´´ from the second and was brighter than the others. The easternmost satellite was 4´ from the third and was smaller than the others ([^63]). The four satellites formed an almost straight line, but the third from Jupiter was slightly above the line. The fixed star, Jupiter, and the easternmost satellite formed an equilateral triangle, as shown in the figure.

March 2: 0 h. 40 m.—Three satellites were in attendance, two on the east and one on the west, in the configuration shown in the diagram ([^64]). The satellite furthest to the east was 7´ from Jupiter; from this satellite the next was distant 0´ 30´´, and the satellite on the west was separated from Jupiter by an interval of 2´. The satellites furthest off were brighter and larger than the remaining satellite, which appeared very small. The satellite furthest to the east seemed to be raised a little towards the north, out of the straight line drawn through the other satellites and Jupiter.

March 2: 0 h. 40 m. Three satellites were visible, two to the east and one to the west, as shown in the diagram ([^64]). The easternmost satellite was 7´ from Jupiter. The next satellite was 0´ 30´´ from the easternmost satellite. The satellite to the west was 2´ from Jupiter. The two satellites farthest from Jupiter were brighter and larger than the remaining satellite, which appeared very small. The easternmost satellite seemed slightly north of the straight line through the other satellites and Jupiter.

The fixed star already noticed was at a distance of 8´ from the western satellite, that is, from the perpendicular drawn from that satellite to the straight line drawn through all the system, as shown in the figure given.

The fixed star mentioned earlier was 8 arcminutes from the western satellite. More precisely, the distance was measured from the perpendicular drawn from that satellite to the line through the whole system. The figure shows this arrangement.

These determinations of the motion of Jupiter and the adjacent planets (his satellites) by reference to a fixed star, I have thought well to present to the notice of astronomers, in order that any one may be able to understand from them that the movements of these planets (Jupiter’s satellites) both in longitude and in latitude agree exactly with the motions [of Jupiter] which are extracted from tables.

I thought it worthwhile to show astronomers these measurements of the motion of Jupiter and the nearby planets (his satellites). These measurements use a fixed star as a reference. I present them so that anyone can understand how these planets (Jupiter’s satellites) move in both longitude and latitude. Their movements agree exactly with the motions [of Jupiter] calculated from tables.

These are my observations upon the four Medicean planets, recently discovered for the first time by me; and although it is not yet permitted me to deduce by calculation from these observations the orbits of these bodies, yet I may be allowed to make some statements, based upon them, well worthy of attention.

These are my observations of the four Medicean planets, which I recently discovered for the first time. Although I cannot yet calculate their orbits from these observations, I may offer some conclusions well worth considering.

And, in the first place, since they are sometimes behind, sometimes before Jupiter, at like distances, and withdraw from this planet towards the east and towards the west only within very narrow limits of divergence, and since they accompany this planet alike when its motion is retrograde and direct, it can be a matter of doubt to no one that they perform their revolutions about this planet, while at the same time they all accomplish together orbits of twelve years’ length about the centre of the world. Moreover, they revolve in unequal circles, which is evidently the conclusion to be drawn from the fact that I have never been permitted to see two satellites in conjunction when their distance from Jupiter was great, whereas near Jupiter two, three, and sometimes all (four), have been found closely packed together. Moreover, it may be detected that the revolutions of the satellites which describe the smallest circles round Jupiter are the most rapid, for the satellites nearest to Jupiter are often to be seen in the east, when the day before they have appeared in the west, and contrariwise. Also the satellite moving in the greatest orbit seems to me, after carefully weighing the occasions of its returning to positions previously noticed, to have a periodic time of half a month.¹⁷ Besides, we have a notable and splendid argument to remove the scruples of those who can tolerate the revolution of the planets round the Sun in the Copernican system, yet are so disturbed by the motion of one Moon about the Earth, while both accomplish an orbit of a year’s length about the Sun, that they consider that this theory of the constitution of the universe must be upset as impossible; for now we have not one planet only revolving about another, while both traverse a vast orbit about the Sun, but our sense of sight presents to us four satellites circling about Jupiter, like the Moon about the Earth, while the whole system travels over a mighty orbit about the Sun in the space of twelve years.

First, the satellites are sometimes behind Jupiter and sometimes ahead of it, at similar distances. They move only a short distance away from Jupiter, either east or west. They also accompany Jupiter whether the planet moves backward or forward. These observations leave no doubt that the satellites revolve around Jupiter. At the same time, all of them together complete twelve-year orbits around the centre of the world. Moreover, the satellites revolve in circles of different sizes. This conclusion clearly follows from the fact that I have never seen two satellites together when far from Jupiter. Near Jupiter, however, I have seen two, three, and sometimes all (four) crowded together. We can also tell that the satellites moving in the smallest circles around Jupiter revolve fastest. The satellites nearest Jupiter often appear in the east after appearing in the west the day before, and vice versa. I have also carefully considered when the satellite in the largest orbit returns to previously observed positions. Its orbital period seems to me to be half a month.¹⁷ Besides, we now have a remarkable and splendid argument to answer some people's objections to the Copernican system. These people can accept that the planets revolve around the Sun. Yet they are deeply troubled by one Moon revolving around the Earth while both orbit the Sun in a year. They therefore believe this theory of the universe's structure must be rejected as impossible. But now we have more than one body revolving around another while both follow a vast orbit around the Sun. Our own eyes show us four satellites circling Jupiter, just as the Moon circles the Earth. Meanwhile, the whole system travels along a mighty orbit around the Sun in twelve years.

Lastly, I must not pass over the consideration of the reason why it happens that the Medicean stars, in performing very small revolutions about Jupiter, seem sometimes more than twice as large as at other times. We can by no means look for the explanation in the mists of the Earth’s atmosphere, for they appear increased or diminished, while the discs of Jupiter and neighbouring fixed stars are seen quite unaltered. That they approach and recede from the Earth at the points of their revolutions nearest to and furthest from the Earth to such an extent as to account for so great changes seems altogether untenable, for a strict circular motion can by no means show those phenomena; and an elliptical motion (which in this case would be nearly rectilinear) seems to be both untenable and by no means in harmony with the phenomena observed. But I gladly publish the explanation which has occurred to me upon this subject, and submit it to the judgment and criticism of all true philosophers. It is certain that when atmospheric mists intervene the Sun and Moon appear larger, but the fixed stars and planets less than they really are; hence the former luminaries, when near the horizon, are larger than at other times, but stars appear smaller, and are frequently scarcely visible; also they are still more diminished if those mists are bathed in light; so stars appear very small by day and in the twilight, but the Moon does not appear so, as I have previously remarked. Moreover, it is certain that not only the Earth, but also the Moon, has its own vaporous sphere enveloping it, for the reasons which I have previously mentioned, and especially for those which shall be stated more fully in my System; and we may consistently decide that the same is true with regard to the rest of the planets; so that it seems to be by no means an untenable opinion to place round Jupiter also an atmosphere denser than the rest of the ether,¹⁸ about which, like the Moon about the sphere of the elements, the Medicean planets (Jupiter’s satellites) revolve; and that by the intervention of this atmosphere they appear smaller when they are in apogee; but when in perigee, through the absence or attenuation of that atmosphere, they appear larger. Want of time prevents my going further into these matters; my readers may expect further remarks upon these subjects in a short time.

Finally, I must consider why the Medicean stars sometimes appear more than twice as large as at other times. These changes occur as they follow their very small orbits around Jupiter. Mists in the Earth’s atmosphere cannot explain the changes. The Medicean stars appear larger or smaller while the discs of Jupiter and nearby fixed stars remain completely unchanged. Nor does it seem at all plausible that changes in their distance from Earth could explain such large differences. A strictly circular orbit could not produce these effects. An elliptical orbit, which in this case would be almost a straight line, seems both implausible and inconsistent with observations. But I gladly offer the explanation that has occurred to me, submitting it to all true philosophers for judgment and criticism. It is certain that atmospheric mists make the Sun and Moon appear larger than they really are. But these mists make fixed stars and planets appear smaller than they really are. Thus, near the horizon, the Sun and Moon appear larger than at other times. Stars, however, appear smaller and are often barely visible. Stars appear even smaller if light illuminates those mists. Thus, stars appear very small during the day and at twilight, but the Moon does not, as I noted earlier. Moreover, it is certain that the Moon, like the Earth, has its own surrounding sphere of vapor. I have already given reasons for this, and will explain others more fully in my System. We may reasonably conclude that the same is true of the other planets. So it seems by no means implausible that Jupiter also has an atmosphere denser than the surrounding ether,¹⁸ around which the Medicean planets (Jupiter’s satellites) revolve. In this respect, they resemble the Moon revolving around the sphere of the elements. Jupiter’s atmosphere would make the satellites appear smaller at apogee, when they are farthest from Earth. But at perigee, when they are nearest Earth, the atmosphere’s absence or reduced thickness would make them appear larger. Lack of time prevents me from exploring these matters further. My readers may expect more discussion of these subjects soon.

Original Configurations of Jupiter’s Satellites observed by Galileo in the months of January, February, and March 1610, and published with the 1st edition of his book Sidereus Nuncius, Venice, 1610.

Original arrangements of Jupiter’s moons observed by Galileo in January, February, and March 1610. Published in the first edition of his book Sidereus Nuncius, Venice, 1610.

FORMING

FORMING