Description: The Galilean satellites are the four biggest and brightest satellites, or moons, orbiting the planet Jupiter: Io, Europa, Ganymede, and Callisto. Closest to Jupiter is Io, which has hundreds of active volcanoes. The main reason for Io's vulcanism is that Io is getting "kneaded" by the tidal effects of Jupiter's gravity. Europa's entire surface is covered with ice. Under the ice is believed to be an ocean of liquid water which is one of the best candidates for harboring life outside of Earth in the Solar System. Ganymede is the largest moon in our Solar System, and also the moon with the largest mass, at twice the mass of Earth's Moon. Callisto is almost exactly the same size as the planet Mercury, but has only about one third of Mercury's mass.

The four Galilean moons were discovered by Galileo Galilei in 1610, as one of several discoveries in Galilei's pioneering campaign of using a telescope for astronomical observations. Galilei was able to document that, over time, the four moons orbit Jupiter like a miniature solar system. This was a clear demonstration that astronomical objects can orbit a center other than the Earth; this observation played an important role in the shifting of the scientific consensus from the Earth-centered (geocentric) to the Sun-centered (heliocentric) view of the Solar System.

Related Terms:

• Io
• Jupiter
• Moons
• Tidal Locking

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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