On January 7, 1610, the Italian astronomer Galileo discovered three of Jupiter's moons: Callisto, Europa and Io. [‘On This Day in Space’ Video Series on Space.com](https://www.space.com/39251-on-this-day-in-space.html)

When he looked at Jupiter through his telescope, he saw what he thought were three tiny stars in the background, and he sketched their locations. The next day, he looked again and saw the three stars were not where he thought they'd be. Instead of being "fixed" in the background like other stars, they moved along with the planet, and Galileo realized that Jupiter had moons. Besides Earth's moon, these were the first natural satellites anyone had ever seen in the solar system.