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viii 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 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