**6. Gravitational theory**

The first approximation of the shape of the earth is the sphere.

The second approximation of the earth is the oblate spheroid.

For theoretically studies and for simple applications it is possible to use the horizontal surface (level surface) or (equipotential surface everywhere, which is perpendicular on the plumb line (force line).

The problem of determine the shape of the earth is actually is the problem of determine the shape of equipotential surface.

The gravitational field include infinity equipotential surfaces.

These equipotential surfaces are not intersect at all.

The scientists deals to considered the sea level as the reference equipotential surface in gravitational studies and call this surface geoid after Listing 1873.

The equipotential surface not necessarily coincide with the equal gravity surface. So the sea level considered approximately a surface of equipotential gravitational surface (because it is perpendicular on the gravity force at every point).
