The four principal natural satellites of Jupiter, discovered by Galileo in 1610: Io (the innermost), Europa, Ganymede, and Callisto. They are distinct worlds in their own right, in the same size range as the Moon. They lack sensible atmospheres and lie in near circular orbits in Jupiter's equatorial plane. Io and Europa are mainly rocky silicate bodies while Ganymede and Callisto contain an equal component of ice. It is conjectured that the four moons were formed by accretion from the material that collapsed to form Jupiter.
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei.
The names that eventually prevailed were chosen by Simon Marius, who claimed to have discovered the moons at the same time as Galileo: he named them after lovers of the god Jupiter: Io, Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614.
Galileo steadfastly refused to use Marius's names and invented as a result the numbering scheme that is still used nowadays, in parallel with proper moon names.
The Galilean moons are, in order from closest to Jupiter to farthest away:
| Name | Image |
Interior structure |
Diameter (km) |
Mass (kg) |
Semi-major axis (km) |
Orbital period (days) |
|---|---|---|---|---|---|---|
| Io | 3643 | 8.93×1022 | 421,800 | 1.77 | ||
| Europa | 3122 | 4.8×1022 | 671,100 | 3.55 | ||
| Ganymede | 5262 | 1.48×1023 | 1,070,400 | 7.16 | ||
| Callisto | 4821 | 1.08×1023 | 1,882,700 | 16.69 |
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