Jupiter Moon Dance

The year was 1610. Galileo, working from his home in Padua, Italy, pointed his homemade spyglass at Jupiter. Night after night, he noticed a line of four stars flanking Jupiter. These stars constantly changed position but never strayed far from the planet.

Galileo had discovered that Jupiter, like the Earth, had moons. The widespread belief that everything revolved around the Earth had just been proven wrong.

In this document, you can see the sketches Galileo drew of Jupiter and its moons as observed through his telescope.

Image courtesy of NASA
Today the four moons discovered by Galileo are known as the Galilean moons. In order of distance from Jupiter, they are Io, Europa, Ganymede and Callisto.

Here is a typical view of Jupiter and its moons, similar to what Galileo would have seen. At the time this photo was taken, Io was hiding behind the planet.

Photo courtesy of NASA/JPL/Malin Space Science Systems
References:

Galileo document
The Discovery of the Galilean Satellites
Wikipedia