The Earth as a Sphere
The earth is spherical in shape, although it is flattened at the North and South poles and wider at the centre around the equator.
The Polar axis: The earth rotates about its axis – a vertical line that runs from the North Pole to the South Pole. The earth takes 24hours to complete a rotation about its axis, which accounts for a day.
The East-West axis: This accounts for the Diameter of the earth and thus the half is the radius R of the earth, the radius of the earth is approximately given as 6,400Km.
The Prime Meridian or The Greenwich Meridian or Zero Meridian: This is an imaginary line drawn from the North Pole to the South Pole to represent 0o meridian from which other meridians are measured either to the East or to the West. This line passes through the town of Greenwich in London-UK and Accra in Ghana. Also, the Prime Meridian divides the earth vertically into equal halves.
The Longitude: These are imaginary lines drawn from the North Pole to the South, but they are measured from the zero meridians to East or to the West i.e. 0o to 180o E or 0o to 180o W.
Equator: The equator is the zero degrees Latitude, it divides the earth horizontally into two equal parts. The other Latitudes are measured from it.
The Latitude: These are imaginary lines drawn from West to East or East to West, but they are measured from the 0o equator to 90o N or from 0o equator to 90o S.
Great Circle: A great circle is a circle that has its centre as the centre of the earth or the sphere it belongs to. All meridians or longitudes and the equator are great circles because they all have their centre as the centre of the earth and thus they have the same radius of the earth i.e. 6400Km.
Small Circle or Parallel of Latitude: All the parallel of Latitude except the equator are small circles, the small circles or parallel of Latitude aside from the equator do not have their centre as the centre of the earth.