7 January 1610 – through his telescope, Italian astronomer Galileo Galilei made the first observation of Jupiter’s Galilean moons: Io, Europa, Ganymede and Callisto, although he was not able to distinguish the first two until the following night.
7 January 1610 – through his telescope, Italian astronomer Galileo Galilei made the first observation of Jupiter’s Galilean moons: Io, Europa, Ganymede and Callisto, although he was not able to distinguish the first two until the following night.
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The Galilean moons (or Galilean satellites) are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in January 1610, and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit a planet other than the Earth.
They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with a radius larger than any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury, though only around half as massive. The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 orbital resonance with each other.
Galileo initially named his discovery the Cosmica Sidera (“Cosimo’s stars”), but the names that eventually prevailed were chosen by Simon Marius. Marius discovered the moons independently at nearly the same time as Galileo, 8 January 1610, and gave them their present names, derived from the lovers of Zeus, which were suggested by Johannes Kepler, in his Mundus Jovialis, published in 1614
The four Galilean moons were the only known moons of Jupiter until the discovery of Amalthea, the “fifth moon of Jupiter”, in 1892.
The start of his beef with the main man himself, pope roman numerals
E pur si muove!