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Basic planetary data

The mean distance of the Earth from the Sun is approximately 1.5 × 108 kilometres. The planet orbits the Sun in a path that is presently more nearly a circle than are the orbits of most other planets. The direction of the Earth's revolutioncounterclockwise as viewed down from the northis in the same sense (direction) as the rotation of the Sun; the Earth's spin, or rotation about its axis, is also in the same direct sense. The length of a day (23 hours, 56 minutes, and 4 seconds) is typical of other planetary objects; Jupiter and most asteroids have days less than half as long, while Mercury and Venus have days more nearly comparable with their orbital periods. The tilt (inclination) of the Earth's axis to its orbit (23.5), also typical, is responsible for the change of seasons.

Compared with the other eight planets of the solar system, the Earth is relatively small. Although it is the largest of the inner planets, it is considerably smaller than the gas giants of the outer solar system. The Earth has a single satellite, the Moon. The Moon is one of the bigger natural satellites in the solar system and is in fact relatively large compared with the Earth itself. Some people consider the Earth-Moon system a double planet, with some similarities to the Pluto-Charon system.

The Earth's gravitational field is manifested as the attractive force acting upon a free body at rest, causing it to accelerate in the general direction of the centre of the planet. Departures from the spherical shape and the effect of planetary rotation cause gravity to vary with latitude over the terrestrial surface. The average gravitational field at sea level is about 980 cm/s2, although values range from about 978 cm/s2 at the Equator to about 983 cm/s2 at the poles.

Gravity typically is not measured at sea level, so corrections must be made for its decrease in value with increasing elevation. Such height-related gravity anomalies may be corrected for by using free-air or Bouguer reductions. In the Bouguer reduction, the effect of the attraction of the additional mass located above sea level is taken into account, while in the free-air reduction this mass effect is ignored. The Bouguer anomaly can be used to indicate variations of density within the Earth by measuring the corresponding variation in gravity.

The Earth's gravity keeps the Moon in its orbit around the planet and also generates tides in the body of the Moon. Such deformations are manifested in the form of slight bulges at the lunar surface, detectable only by sensitive instruments. The Moon, owing to its relatively large mass, exerts a gravitational force that likewise causes tides on the Earth. These are most readily observable as the daily rises and falls of the ocean water, although tidal deformations occur in the solid Earth as well as in its atmosphere.


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THE EARTH | The atmosphere and hydrosphere

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