The Moons of Jupiter

By | Jan 7, 2011

In a letter dated January 7th, Galileo Galilei wrote about a discovery of his of what he thought were three fixed stars near Jupiter. Soon he found that there were four and they weren’t stars but bodies that were in orbit around the planet.

Even though these four moon are bright, they would not have been seen without the aid of a telescope. They could be seen if they were farther away from Jupiter.

Galileo initially called his discovery the Cosmica Sidera (“Cosimo’s stars”), in honour of Cosimo II de’ Medici (1590–1621). At Cosimo’s suggestion, Galileo changed the name to Medicea Sidera (“the Medician stars”).

The received their final names Io, Europa, Ganymede, Callisto by Simon Marius after the lovers of the Greek god Zeus. Zeus is the equivalent of to the Roman god Jupiter. Galileo refused to use these names and just simply used a numbering scheme listing them from closest to Jupiter outward as I, II, III, IV.

The Galilean moons would be considered dwarf planets if they were in direct orbit around the sun and not also orbiting Jupiter. The are spheroidal in shape.

If one counts a recently discovered moon that hasn’t established an orbit, Jupiter has 63 moons. Jupiter has eight moons that are termed regular satellites. This include the Galilean moons. The other 55 are classified as irregular satellites with 14 of these are still unnamed.


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