Note: This is an edited repost of an article from 2014.
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!
As a result of his research and publications regarding the (actual) motions of bodies in the solar system, Galileo was eventually sentenced as a heretic by the Inquisition in Rome and spent the last 9 years of his life under house arrest. But his legacy of observation, logic, and science over dogma and religious control lives on to this day… in fact, if you go outside on a clear night now you can see a brightly shining Jupiter high in the eastern sky, just as Galileo did 404 years ago. Take even a small telescope or pair of binoculars and you will easily see its four largest moons as pinpoints of light beside it. Thanks to Galileo and others like him, we now know what those are, and that there are countless other worlds and moons out there like them, just waiting for discovery.
*Eppur si muove – “and yet it moves” is a quote often attributed to Galileo, in that he muttered it as he “recanted” and accepted his punishment by the Roman court. But it is likely purely apocryphal, as there is no mention of it in records from the time.