Galileo Galilei First Observes Jupiter's Moons

Galileo Galilei First Observes Jupiter's Moons

Galileo Galilei
Source: Wikipedia

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While conducting observations with his telescope, Galileo Galilei sees what he describes as "three fixed stars, totally invisible by their smallness." Subsequent observations reveal that the points of light move in ways that are inconsistent with stars, but which make sense if they are moons.

And in fact they are four of Jupiter's moons: Ganymede, Callisto, Io and Europa. They will be named the Galilean Moons in his honor.

This observation of extraterrestrial moons will spark a revolution for Galileo because the Aristotelian Cosmology defended by the Catholic Church requires that all things in heaven orbit the earth, so it's impossible for anything in the heavens to have satellites.

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