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in 1610.

Galileo Galilei makes his first observation of the four Galilean moons: Ganymede, Callisto, Io & Europa, although he is not able to distinguish the last two until the following night.

His work Sidereus Nuncius contains the results of his early observations of the imperfect and mountainous Moon, of hundreds of stars not visible to the naked eye in the Milky Way and in certain constellations, and of the Medicean Stars (later Galilean moons) that appeared to be circling Jupiter.
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Title page of Sidereus nuncius, 1610, by Galileo Galilei (1564-1642).
Title page of Sidereus nuncius, 1610, by Galileo Galilei (1564-1642). The title, Sidereus Nuncius, appears prominently, often in bold or larger font, followed by a subtitle or additional descriptive text explaining the nature of the work. It mentions Galileo's role as the author and his academic affiliations, emphasizing his authority as a mathematician and philosopher. The publisher’s information is typically located at the bottom of the page. It identifies the place of publication (Venice) and the printer (Tommaso Baglioni), adding to the historical value of the artifact. The title page represents a monumental scientific achievement. Sidereus Nuncius detailed Galileo's observations of the Moon's rugged surface, the discovery of Jupiter's four largest moons, and the vast number of stars unseen by the naked eye, fundamentally altering humanity’s understanding of the cosmos. The simplicity of the title page belies the revolutionary content within.