On Jan. 7, 1610, Galileo Galilei turned his self-built telescope, which he had recently made improvements to, towards Jupiter. There he noticed three spots of light. Though he first concluded they were distant stars, he returned to observing the bodies on subsequent nights, and realized the motion of the “stars” didn’t track with the other stars around them. They also followed a specific pattern relative to Jupiter (though they changed position relative to each other) and never left the planet’s vicinity.
Four days after he spotted the first three points of light, Galileo observed a fourth that followed the same behavior. By Jan. 15, he had realized he was looking at moons orbiting Jupiter, not at stars. His observations provided early evidence to support Nicolaus Copernicus’ theory that the solar system was not geocentric – i.e., it did not revolve around the Earth. Galileo’s findings were published in March of 1610, in his book Siderius Nuncius (The Starry Messenger). Though Galileo called the moons the “Medicean Stars” (after his patrons), Johannes Kepler suggested they be named Io, Europa, Ganymede, and Callisto, the names we know them by today.
