Note: This is an edited repost of an article from 2014.
Jupiter and the four “Galilean moons” Europa, Io, Callisto, and Ganymede. (Jan Sandberg/www.desert-astro.com)
405 years ago tonight, January 7, 1610, the Pisan astronomer Galileo Galilei looked up at a bright Jupiter at opposition through his handmade telescope and saw three little “stars” next to it, which piqued his natural scientific curiosity. He soon realized that these little objects weren’t stars at all but rather moons orbiting the giant planet (and, most importantly, not Earth). Further observations over the next few nights showed that the planet wasn’t moving relative to the little “stars” as it should if they were indeed background stars, and in fact the smaller bodies (of which he soon saw four) were moving along with Jupiter each in its own little orbit. This revelation helped change our entire view of the Solar System, causing no end of trouble for Galileo (as the Church didn’t appreciate data contradicting their conveniently Earth-centered Universe) but also opening the door for the discovery of many more moons around other planets.
Today Jupiter is now known to have at least 50 moons, with possibly as many as 67!
Drawings from Galileo’s notebook