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Planets and moons

Galilean moons

The four Galieans to scaleThe four Galieans to scale Jupiter’s four largest moons (it has 67 in all) are called the “Galileans” after the Italian scientist Galileo Galilei (1564–1642) who discovered them with one of the first telescopes in 1610. Until he saw that they moved around Jupiter, Galileo's first thought was that they were stars. Galileo called his discoveries the Medicea Sidera ("the Medician stars"), in honour of the Medici family, who ruled Florence at the time. They were given their current names four years later. All the Galilean moons are larger than the dwarf planet Pluto, and Ganymede is larger than Mercury.

What the surface of Io could look likeWhat the surface of Io could look like



Ganymede, at 5268 kilometres (3273 miles) across, is the largest moon in the Solar System. It also has the greatest mass of any moon, more than twice that of Earth's Moon. It is the only moon known to have its own strong magnetic field. Ganymede has an icy surface with dark, cratered plains and areas showing strange “grooved” patterns, as if someone has clawed away at its surface with a giant fork. Scientists believe there might be a water ocean about 200 kilometres (125 miles) below Ganymede's surface, trapped between layers of ice.



Galileo's discovery of the moons orbiting a world, Jupiter, other than the Earth, shattered the theory held at the time (1610) that everything in the Universe orbited around the Earth.

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