On January 7, 1610, Italian astronomer Galileo Galilei discovered, using a homemade telescope, four moons orbiting the planet Jupiter. Looking at what he thought were a group of stars, he realized the objects appeared to move in a regular pattern. These objects moved in the "wrong direction," according to the understanding of nature at the time. After a few weeks, Galileo determined that he was observing not stars, but objects in orbit around Jupiter. Today, Jupiter’s four largest satellites—Io, Europa, Ganymede, and Callisto—are named the Galilean Moons in honour of their discoverer.

Galileo’s discovery provided evidence for the Copernican understanding of the universe. This was the idea that everything in existence did not, indeed, move around Earth. His discoveries led to the development of modern astronomy.