Longitude

Imaginary lines running north/south at right angles to the parallels and passing through the poles are known as lines of longitude or meridians. The line of longitude passing through Greenwich (London) is 0 degrees or the prime meridian (so called because all lines of longitude are numbered east or west from it). The longitude of a place is its angular distance east or west of the Greenwich meridian, measured at the centre of the earth. There are 180 degrees of west longitude and similarly 180 degrees of east longitude. However, since there are 360 degrees in a circle. 180 degrees East and 180 degrees West must be one and the same line. Since the earth is spherical and has a circumference calculated at 40,232.5 km, in linear distance each of the 360 degrees of longitude is 40,232.5/360 or 111.757 km. As the parallels of latitude become shorter polewards, so the meridians of longitude, which coverage at the poles, enclose a narrower space. The degree of longitude therefore decreases in length. It is longest at the equator where it measures 111.318 km. At 25 degrees it is 100.95 km; at 45 degrees it is 78.856 km; at 75 degrees, 28.967 km; and at the poles 0 km. There is so much difference in the length of degrees of longitude outside the tropics, that they are not used for calculating distances as in the case of latitude. But they have one very important function, they determine local time in relation to G.M.T. or Greenwich Mean Time, which is sometimes referred to as World Time.
THE POSITION OF A PLACE
It is necessary to be precise in stating the position of a place in degrees, since there are two latitudes X degrees (X degrees North and X degrees South). Similarly, longitude Y degrees refers to either opposite meridians unless we state it as Y degrees east and Y degrees west.