Sets of lines drawn from pole to pole in a north-south direction are called meridians of longitude. These lines cut the latitudes at right angles. They are semicircles. Due to the shape of the earth, the distance between two latitudes is greatest at the equator and decreases pole-wards till they all meet at the North and South Poles.
Longitudes are also measured in degrees. They are further divided and subdivided into ‘minutes’ and ‘seconds’ for more accurate location. Unlike parallels of latitude, all meridians of longitude are equal in length. For convenience, it was internationally decided in 1884 to accept the meridian that passes through the Greenwich village in England (where the British Royal Observatory is located) as the 0o longitude o prime meridian. It was agreed that the counting of meridians would begin from that place. A metal marker has also been placed there.
The angular distance of a place, east or west of the prime meridian is called longitude. There are 360 longitudes drawn through the equator at equal intervals. Those that lie to the east of the prime meridian are marked with the letter ‘E’, e.g., 0o – 180o E and those to the west of the prime meridian by the letter ‘W’ e.g. 0o – 180o W. The longitude of 180 is called the international date-line. It is followed neither by ‘E, no by ‘W’. The 0o and 180o longitude together make a full circle around the earth. Similarly any longitude joined with the one opposite to it will form a Great Circle. For e.g., 30o W and 150o also form a Great Circle. All Great Circles are equal.
It is with the help of the latitudes and longitudes that a place can be accurately located on a map or a globe – it may be a country, city, a town or a village. Given below are the latitudes and longitudes of a few places. Try to find them in the map in an atlas.