Galilean Moons Day - January 7

On this date in 1610 Galileo Galilei began to tell people about a discovery he made with his latest refracting telescope: celestial bodies around the planet Jupiter that turned out to be the Galilean moons. These are the 4 largest moons of Jupiter, Io, Europa, Ganymede, and Callisto, and are the largest objects in our solar system outside of the Sun and the 8 planets.

It sounds like he wasn't certain what to make out of them at first, originally naming them the Medicean stars. When he realized that they were actually moons of Jupiter, the ramifications were enormous. Despite Copernicus proposing a heliocentric solar system, at the time of this discovery the prevailing theory was that everything in the sky orbited the Earth. Finding moons that orbited Jupiter was the first evidence that a geocentric understanding of the universe was flawed.

Today we celebrate this astronomic discovery. (Composite photo below.)

NASA/JPL/DLR, Public domain, via Wikimedia Commons