The earth is a concept that is closest to the shape of the modern earth. Suppose that in calm ocean waters, seawater will form a stable surface under the action of gravity. This surface is perpendicular to the direction of gravity and the gravity is equal everywhere, that is, the gravity potential surface. If this imaginary surface extends below the mainland, it constitutes a globally closed approximate sphere, which is the surface of the globe.

A globe is defined as a sphere surrounded by the geoid, which is the true shape of the earth. According to satellite observation, the northern and southern hemispheres of the earth are asymmetrical, with the North Pole protruding and the South Pole depressed; The mid-latitude southern hemisphere is convex and the northern hemisphere is concave, with irregular shape.

Extended data:

The ellipsoid of the earth is also called "the oblate sphere of the earth". A mathematical surface representing the size and shape of the earth. Represented by long radius and oblateness. Because it is very close to the ellipsoid, the reference ellipsoid is usually used to represent the shape and size of the earth ellipsoid. A figure formed by an ellipse rotating around its short axis is similar to the geoid of the earth. The shape of the geoid is expressed by the deviation from the reference ellipsoid.

Generally speaking, the shape and size of the earth are actually expressed by the semi-major diameter, semi-minor diameter and oblateness of the reference ellipsoid. The data recommended by the International Federation of Geodesy and Geophysics 1975 are: semi-long diameter 6378 140m, semi-short diameter 6356755m, flatness 1∶298.257. Generally speaking, it is a smooth and regular globe formed by the rapid rotation of the earth around the short axis. It is a description of the shape of the earth and is defined for the calculation of measurement results and the need of drawing work.

References:

Baidu Encyclopedia-Globe, Baidu Encyclopedia-Earth ellipsoid