# Definitions at Earth

## Shape:

In reality, the shape of the earth is elliptic, slightly bulked around the equator and slightly flattened in the poles.
In navigation, earth is considered to be a sphere given that the errors caused by this assumption are negligible.

## The Movement of the Earth

The earth, being a spherical isolated body in space spins continuously with the same speed around its axis and completes a full spin in 24 hours. At the same time, earth moves around the sun and completes a full spin (elliptical) in 365 days.

## The Axis of the Earth

Is the axis around which the earth spins. It penetrates its surface in two opposite points that are called ‘poles of the Earth’.
The pole P that is situated near the polar star is called ‘north pole’. The pole P’ that is situated near the side of the Southern Cross star is called ‘south pole’.

## Equator

Is the great circle whose plane is perpendicular to the axis of the Earth and divides the Earth into two hemispheres, the North and the South.

## Great circles

Is the circle defined by the intersection of the surface of the Earth and every plane that passes through the center of the Earth.

## Parallels

Are the small circles, whose plane is parallel to the plane of the equator and vertical to the axis of the earth.
A single parallel passes through one and only place on earth. Parallels are countless.

## Meridian

Is the great circle on the surface of the Earth that passes through the poles. A single meridian perforates one and only place on earth.

## Prime Meridian

The meridian that runs through Britain’s Greenwich observatory is called ‘prime meridian’ and divides the Earth in two hemispheres the eastern and the western.