It is an imaginary line that runs from the North Pole to the South Pole and on her land develops its rotational movement, this imaginary line is not perpendicular to the plane of its movement of travel around the Sun but that it has a penchant, already known by the Chinese from the 1,104 year astronomers a. d. C., 23,43 degrees, which makes the rays of the Sun not affect in the same way upon the Earth in its orbit around Sun