Introduction

In 1609, Galileo, then Professor of Mathematics at Padua, in the service of the Venetian Republic, heard from a correspondent at Paris of the invention of a telescope, and set to work to consider how such an instrument could be made. The result was his invention of the telescope known by his name, and identical in principle with the modern opera-glass. In a maritime and warlike State, the advantages to be expected from such an invention were immediately recognised, and Galileo was rewarded with a confirmation of his Professorship for life, and a handsome stipend, in recognition of his invention and construction of the first telescope seen at Venice. In his pamphlet, The Sidereal Messenger, here translated, Galileo relates how he came to learn the value of the telescope for astronomical research; and how his observations were rewarded by numerous discoveries in rapid succession, and at length by that of Jupiter’s satellites. Galileo at once saw the value of this discovery as bearing upon the establishment of the Copernican system of astronomy, which had met with slight acceptance, and indeed as yet had hardly any recommendation except that of greater simplicity. Kepler had just published at Prague his work on the planet Mars (Commentaria de motibus Stellæ Martis), on which he had been engaged apparently for eight years; there he heard of Galileo’s discoveries, and at length was invited by Galileo himself, through a common friend, Giuliano de’ Medici, ambassador of the Grand-Duke of Tuscany, Cosmo de’ Medici II., to the Emperor Rudolph II., to correspond with Galileo on the subject of these discoveries. The Emperor also requested his opinion, and Kepler accordingly examined Galileo’s Sidereal Messenger in a pamphlet, entitled A Discussion with the Sidereal Messenger (Florence, 1610).

In 1609, Galileo was Professor of Mathematics at Padua, serving the Venetian Republic. A correspondent in Paris told him about the invention of a telescope. Galileo then began considering how to make such an instrument. The result was his invention of the telescope that bears his name. Its operating principle is identical to that of modern opera glasses. In a state focused on seafaring and warfare, the expected advantages of such an invention were immediately recognized. Galileo was rewarded with a lifetime appointment to his professorship and a generous salary. These rewards recognized his invention and construction of the first telescope seen in Venice. In his pamphlet, The Sidereal Messenger, translated here, Galileo explains how he discovered the telescope’s value for astronomical research. He describes how his observations led to many discoveries in rapid succession, culminating in the discovery of Jupiter’s satellites. Galileo immediately recognized the discovery’s value in helping establish the Copernican system of astronomy. That system had gained little acceptance and, so far, had hardly anything to recommend it except greater simplicity. Kepler had just published his work on the planet Mars (Commentaria de motibus Stellæ Martis) at Prague. He had apparently spent eight years working on the book. While in Prague, Kepler heard about Galileo’s discoveries. Eventually, Galileo himself invited Kepler to correspond with him about the discoveries through their mutual friend, Giuliano de’ Medici. Giuliano was ambassador to Emperor Rudolph II. for the Grand-Duke of Tuscany, Cosmo de’ Medici II. The Emperor also asked for Kepler’s opinion. Kepler therefore examined Galileo’s Sidereal Messenger in a pamphlet titled A Discussion with the Sidereal Messenger (Florence, 1610).

In this Discussion Kepler gives reasons for accepting Galileo’s observations—although he was not able to verify them from want of a telescope—and entirely supports Galileo’s views and conclusions, adducing his own previous speculations, or pointing out, as in the case of Galileo’s idea of earth-light on the moon, the previous conception of the same explanation of the phenomenon. He rejects, however, Galileo’s explanation of the copper colour of the moon in eclipses. Kepler ends by expressing unbounded enthusiasm at the discovery of Jupiter’s satellites, and the argument it furnishes in support of the Copernican theory.

In this Discussion Kepler gives reasons for accepting Galileo’s observations, although he could not verify them without a telescope. He fully supports Galileo’s views and conclusions, citing his own earlier speculations or noting that an explanation had been proposed before. One example is Galileo’s idea that light from Earth illuminates the moon. However, Kepler rejects Galileo’s explanation of the moon’s copper colour during eclipses. Kepler ends by expressing boundless enthusiasm for the discovery of Jupiter’s satellites and its support for the Copernican theory.

Soon after, in 1611, Kepler published another pamphlet, his Narrative, giving an account of actual observations made in verification of Galileo’s discoveries by himself and several friends, whose names he gives, with a telescope made by Galileo, and belonging to Ernest, Elector and Archbishop of Cologne. Kepler and his friends saw the lunar mountains and three of the satellites of Jupiter, but failed to make out any signs of the ring of Saturn corresponding to the imperfect description of Galileo.

Soon after, in 1611, Kepler published another pamphlet, his Narrative. It described observations he and several friends had made to verify Galileo’s discoveries. Kepler named these friends in the pamphlet. They used a telescope made by Galileo and owned by Ernest, Elector and Archbishop of Cologne. Kepler and his friends saw the mountains on the Moon and three of Jupiter’s satellites. But they could not see any signs of Saturn’s ring matching Galileo’s incomplete description.

Kepler had previously published a treatise on Optics (Frankfort, 1604). He now extended it to the consideration of the theory of the telescope, and explained the principle of Galileo’s telescope; he also showed another combination of lenses which would produce a similar effect. This was the principle of the common astronomical telescope, often called, from this circumstance, Kepler’s telescope, though he did not construct it. The account of Galileo’s later astronomical discoveries of Saturn’s ring and the phases of Venus is taken from the preface of this work.—(Kepler’s Dioptrics; Augsburg, 1611.)

Kepler had previously published a treatise on Optics (Frankfort, 1604). He now expanded his work to cover the theory of the telescope and explained how Galileo’s telescope worked. He also showed another combination of lenses that would produce a similar effect. This combination became the basis of the standard astronomical telescope, often called Kepler’s telescope for this reason. Kepler did not, however, build it. The account of Galileo’s later astronomical discoveries, Saturn’s ring and the phases of Venus, comes from this work’s preface. (Kepler’s Dioptrics, Augsburg, 1611.)

In 1612 Galileo published a series of observations of solar spots, and in 1618 some observations of three comets. There exist also long series of minute observations of Jupiter and his satellites, continued to November 1619.—(Galileo’s Works; Florence, 1845.)

In 1612 Galileo published a series of observations of sunspots, and in 1618 some observations of three comets. Long series of detailed observations of Jupiter and its satellites also survive, continuing through November 1619. (Galileo’s Works, Florence, 1845.)

The following editions have been used for the translation:—

The following editions were used for this translation:

Galileo’s Works.

Galileo’s Works.

Sidereus Nuncius.

The Starry Messenger.

Kepler’s Works, ed. C. Frisch. Frankfurt a. M., 1858-71.

Kepler’s Works, edited by C. Frisch. Frankfurt am Main, 1858 to 1871.

THE

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SIDEREAL MESSENGER

STARRY MESSENGER

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GALILEO GALILEI

GALILEO GALILEI

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