Earth : one of the planets in the solar system, the third in distance from the sun and the fifth largest of the planets in diameter. The mean distance of the earth from the sun is 149,503,000 km (92,897,000 mi). It is the only planet known to support life, although some of the other planets have atmospheres and contain water.

The earth is not a perfect sphere but is slightly oblate, or flattened at the poles. The diameter of the earth measured around the North Pole and the South Pole is about 42 km (26 mi) less than the diameter of the earth measured at the equator.


II. Motion


In common with the entire solar system, the earth is moving through space at the rate of approximately 20.1 km/sec or 72,360 km/h (approximately 12.5 mi/sec or 45,000 mph) toward the constellation of Hercules. The Milky Way galaxy as a whole, however, is moving toward the constellation Leo at about 600 km/sec (about 375 mi/sec). The earth and its satellite, the moon, also move together in an elliptical orbit about the sun. The eccentricity of the orbit is slight, so that the orbit is virtually a circle. The approximate length of the earth's orbit is 938,900,000 km (583,400,000 mi), and the earth travels along it at a velocity of about 106,000 km/h (about 66,000 mph). The earth rotates on its axis once every 23 hr 56 min 4.1 sec (based on the solar year). A point on the equator therefore rotates at a rate of a little more than 1600 km/h (about 1000 mph), and a point on the earth at the latitude of Portland, Oregon (45° north), rotates at about 1073 km/h (about 667 mph).

In addition to these primary motions, three other components of the total motion of the earth exist: the precession of the equinoxes , nutation (periodic variation in the inclination of the earth's axis caused by the gravitational pulls of the sun and moon), and variation of latitude .