The earth is an irregular ellipsoid with slightly flat poles. The earth rotates from west to east and revolves around the sun at the same time. The combination of the earth's rotation and revolution has produced the alternation of day and night and seasonal changes on the earth (the speed of the earth's rotation and revolution is uneven). At the same time, due to the gravity of the sun, the moon and nearby planets, as well as the influence of many factors such as the earth's atmosphere, oceans and materials inside the earth, the direction of the earth's rotation axis will change in space and the earth itself. The inertial centrifugal force produced by the earth's rotation makes the spherical earth gradually expand from the poles to the equator and become a slightly flat rotating ellipsoid at present. The polar radius is about 2 1km shorter than the equatorial radius, with a circumference of 40,000 km and a surface area of 5 1 10,000 square km. At present, the best estimate of the age of the earth is 4.55 billion years. The age of the earth usually refers to its astronomical age. The astronomical age of the earth refers to the time from the beginning of the earth to the present, which is closely related to the hypothesis of the origin of the earth.

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After long-term accurate measurement, scientists found that the earth is not a regular sphere, but an irregular elliptical sphere with slightly flat poles and slightly bulging equator. Exaggerated, it is a bit like a "pear", called "pear shape". [3] The equatorial radius of the earth is about 6378. 137Km, which is very small compared with the average radius of the earth. From the perspective of space, the earth can still be regarded as a regular sphere. If you make a globe with a radius of 1 m according to this ratio, the equatorial radius is only about 3 mm longer than the polar radius, which is difficult for human eyes to detect, so when making a globe, it is always made into a regular sphere.