Great and Small Circles

The horizontal plane, through the centre of the earth, and perpendicular to the axis of the Earth, forms a boundary line which is a circle. This circle is called a Great circle. There are other Great Circles, but this particular Great Circle is called the Equator. Any horizontal plane, not through the centre of the earth but perpendicular to the axis of thee earth, will also form a boundary line which is a circle. Such a circle is however called a small circle or a parallel of latitude or simply a latitude. With the exception of the Equator, all  the parallel of the latitudes are small circles, because they are formed by horizontal planes which do not pass through the centre of the Earth.

The Equator is taken as a reference latitude. Latitudes  Above the equator are said to be north of the Equator, while latitudes below the equator are said to be south of the equator.

A vertical plane through the axis of the earth will also form a boundary line which is a great circle. This Great circle is called a meridian. Half a meridian is called a longitude.

Just as we have a reference latitude, so also do we have a reference longitude called, the prime meridian.  Longitudes to the left of the prime meridian are said to be West of the prime Meridian, while longitudes to the right are said to be East of the Prime meridian.

All Meridians are Great Circles. The prime meridian is designated as longitude O Degree since it is a reference longitude it passes through the city of tema in Ghana, and a place in London called Greenwich, and for this reason, it is sometimes called the Greenwich Meridian.

The Equator also being a reference latitude is designated ad latitude O Degree. The position of any place on the Earth’s surface is unique determined if we state the angles of its latitude and longitude.

In figure Above A , B G is the Equator, E A F  is a parallel of Latitude north of the Equator while I C J is a parallel of Latitude of south of the Equator. 

The Merideian NABCS cuts the parallel of latitude E A F at A the Equaator at B and the parallel of latitude I C J at C.

The minor arc AB subtends angle AOB = a at the centre of the earth and because of this the point A is said to have a latitude a degree north of the Equator. Similarly, the minor arc BC  subtends angle COB = b at the centre of the Earth and because of this, the point C is said to have a latitude b degree south of the Equator.