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.