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The Discovery of New Moons

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Galileo made these sketches of the moons of Jupiter, as seen through his telescope, on the back of an envelope. The sketches record observations on different nights during the period January 14 to 25, 1610. It looked to him like stars shifting back and forth around the planet.

What he saw are the four larger moons of Jupiter, now known as Io, Europa, Ganymede and Callisto. Galileo had considerable difficulty in recognizing the true meaning of what he was seeing; Callisto often lay outside the (restricted) field of view of his telescope, Io was often lost in Jupiter's glare, and some moons occasionally disappeared in Jupiter's shadow or behind or in front of the planet itself.

Galileo named the moons "Medicean Stars," after the ruling Florentine Medici family. This was a move calculated to improve his chances of moving back to Florence, and it succeeded. The names used today were coined by Simon Mayr (1573-1624), who for a time claimed priority on their discovery.
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