One of the most important contributions Galileo has made to science was his discovery of the four moons that orbit around Jupiter. Galileo first observed the moons of Jupiter, which are now named in his honor, 400 years ago.
By the end of 1609, Galileo had perfected his primitive "spy glass," which he unveiled in the summer, into a scientific instrument. Although his telescope had a small lens with poor resolution, he began making astonishing discoveries.
On Jan. 7, 1610, Galileo turned his 30-power telescope toward Jupiter. The telescope's small lens and narrow field of view made it difficult for him to find Jupiter, but when he did, he was amazed to discover that Jupiter had moons whizzing around it.
He originally thought he saw three stars near Jupiter, strung out in a parallel line with the planet. However, on his next evening of observation, these stars seemed to move. Galileo continued to observe the stars and Jupiter for the next week. On Jan. 13, a fourth star appeared. After a few weeks, Galileo had observed that the four stars never left the vicinity of Jupiter and appeared to move with the planet, and that they changed their position with respect to each other and Jupiter.
Galileo eventually determined he wasn't observing stars but planetary bodies that were in orbit around Jupiter. This discovery provided evidence in support of the Copernican system and showed that everything did not revolve around Earth.
Galileo may not have been the first person to observe Jupiter's moons. An astronomer named Simon Marius claimed to have observed Jupiter's moons about five weeks prior to Galileo. However, because Marius did not publish his observations right away as Galileo had done, his claims were impossible to verify.
Marius did, however, provide the names of Jupiter's moons, Io, Europa, Ganymede and Callisto, based on a suggestion from Johannes Kepler.