It was on this date in 1610 that Galileo Galilei turned his primitive telescope on Jupiter and shattered the notion that everything orbits the Earth. Having recently constructed a telescope that he had based off of a Dutch design, Galileo turned it on the heavens and made discoveries that re-shaped the universe.
At the time, it was widely believed that everything orbited the Earth, a belief held virtually unchallenged since Classical Greece and only recently challenged (in theory only) by Copernicus. Turning his telescope in Jupiter, Galileo noted 4 small stars near the planet, nothing unusual. However, in the coming nights, Galileo noticed that the 'stars' moved relative to each other while remaining with the planet. The only conclusion: the 'stars' were moons. Now known as the Galilean Moons, they are, in order from Jupiter: Io, Europa, Ganymede (largest moon in the solar system), and Callisto. An easy way to remember this fact: I(Io) Eat(Europa) Green(Ganymede) Caterpillars(Callisto)!