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Galilean moon

Jupiter's 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). From the top, they are Io, Europa, Ganymede and Callisto.

The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. By far the largest of the many moons of Jupiter, they were visible with the low-power telescope he invented. They were first observed by Galileo on January 7, 1610.

Galileo observed the moons' motion over several days and realized that they were in orbit around Jupiter. This discovery supported the heliocentric theory of Copernicus and showed that not everything revolves around the Earth.

In order from closest to Jupiter to farthest away, they are:

Group Name Diameter (km) Mass (kg) Mean orbital
radius (km)
Orbital period
2 Io 3632 8.92×1022 421,600 1.76 days
Europa 3138 4.8×1022 670,900 3.55 days
Ganymede 5262 1.49×1023 1,070,000 7.16 days
Callisto 4820 1.08×1023 1,883,000 16.69 days

See also: Jupiter's moons