Jupiter is the largest of the nine planets that travel around the Sun. Its orbit lies out beyond those of Mars and the asteroid belt, at an average distance from the Sun of 777 million kilometers (483 million miles). It takes 11.86 Earth years to complete one orbit and rotates on its axis once every 9 hours 55 minutes 29 seconds.

From Earth, Jupiter appears as a bright disk second only to Venus in brightness. Ancient astronomers called it Jupiter in honor of the ruler of the gods worshipped in the Greco-Roman world, known in Greek as Zeus. But at the time they had no idea how suitable this name was. In fact, Jupiter is larger than all the other planets put together. Its mass is 318 times as great as that of the Earth, and its diameter is 142,800 kilometers (88,730 miles), 11 times that of the Earth. Jupiter's surface gravity is nearly three times that experienced on Earth. It would take more than 1,500 Earths to fill up the space occupied by Jupiter. But Jupiter is, comparatively speaking, a remarkably light planet; its density is 1.33 grams per cubic centimeter, only just above that of water, which is 1 g/cm3.

The Satellites of Jupiter
Scientists have so far discovered 16 moons circling Jupiter. The four largest, named Ganymede, Callisto, Io, and Europa were discovered by the Italian astronomer Galileo in 1610 and are thus known as the Galilean satellites. Ganymede, the largest, with a diameter of 5,276 kilometers (3,279 miles), is larger than the planet Mercury.

The Pull of Jupiter
Because of its position as the largest planet in the Solar System, Jupiter's gravitational effect on other bodies is very strong. Some of its moon may well be asteroids captured from their orbits round the Sun.

1988 Encyclopaedia Britannica, Inc.

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