When Galileo pointed his telescope at Jupiter in 1610, he made a groundbreaking discovery. He saw four bright objects near the planet that moved in relation to it. These objects were, of course, Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto.

Here's what Galileo observed:

* They moved: The four objects were not stationary stars but seemed to orbit Jupiter. This challenged the prevailing geocentric view of the universe, where everything revolved around Earth.

* They were not stars: Galileo was able to distinguish the moons from stars because they were much closer to Jupiter and appeared to move relative to it.

* They were not just a trick of the telescope: Galileo observed the moons on multiple nights and confirmed their movement.

Galileo's observations of Jupiter's moons were a major contribution to the Copernican revolution and provided strong evidence for the heliocentric model, where the planets revolve around the sun.