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#OTD 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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