Like all planets of the Solar system, the Earth has a spherical shape. Before talking about its exact dimensions, we introduce several important geographic concepts.

The earth rotates around an imaginary straight line - the so-called **earth's axis**. The point of intersection of the earth's axis with the earth's surface are called **poles**. Two: North and South. The line of intersection of the surface of the globe by plane passing through the center of the Earth perpendicular to the earth's axis, is called **the equator**. Plane that intersects the earth's surface parallel to the plane of the equator, forming **parallel**, and the plane passing through the two poles - **the meridians**.

Due to the rotation around its axis and the resulting centrifugal force, the Earth is slightly flattened near the poles and the semi-major axis (Equatorial radius, r_{c}) almost 21.4 miles more than the distance from the center of the Earth to the poles. Such flattened at the poles of the ball is called **a spheroid** or **ellipsoid of rotation**.

In Russia for geodetic and cartographic works using the ellipsoid F. N. Krasovsky (named after the scientist, under whose guidance was carried out calculations). Its dimensions are as follows:

- Equatorial radius - 6378,2 km,
- the polar radius is 6356,8,
- the length of the Meridian - 40008,5 km,
- the length of the equator - 40075,7 km,
- the surface area of the Earth is 510 million km
^{2}.

In fact, the figure of the Earth is even more difficult. It deviates from the correct shape of a spheroid due to the inhomogeneous structure of the subsoil, and the uneven distribution of mass. The true geometric shape of the Earth is called **the geoid** ("earth"). The geoid is the shape, the surface of which is everywhere perpendicular to the direction of gravity, i.e. plumb.

The surface of the geoid coincides with the reference surface of the World ocean (mentally continued under the continents and Islands). Raising and lowering of the geoid above the ellipsoid are 500-100 m

The physical surface of the Earth, is complicated by the mountains and valleys does not coincide with the surface of the geoid, retreating from him for several kilometers. Gravity is constantly striving to align the surface of the Earth, to bring it into conformity with the surface of the geoid.