Galileo, on January 7, 1610, observes with…

Galileo, on January 7, 1610, observes with his telescope what he describes at the time as "three fixed stars, totally invisible by their smallness," all close to Jupiter, and lying on a straight line through it.

Observations on subsequent nights show that the positions of these "stars" relative to Jupiter are changing in a way that would be inexplicable if they were really fixed stars.

Galileo notes on January 10 that one of them has disappeared, an observation which he attributes to its being hidden behind Jupiter.

Within a few days he concludes that they are orbiting Jupiter: He has discovered three of Jupiter's four largest satellites (moons).

He discovers the fourth on January 13.

These satellites are now called Io, Europa, Ganymede, and Callisto.

Galileo named the group of four the Medicean stars, in honor of his future patron, Cosimo II de' Medici, Grand Duke of Tuscany, and Cosimo's three brothers.

Later astronomers, however, will rename them Galilean satellites in honor of their discoverer.

Once Galileo realizes what he has seen a few days later, his observations of the satellites of Jupiter create a revolution in astronomy that reverberates to this day: a planet with smaller planets orbiting it does not conform to the principles of Aristotelian Cosmology, which holds that all heavenly bodies should circle the Earth, and many astronomers and philosophers initially refuse to believe that Galileo could have discovered such a thing.