Longitude is the distance of a place east and west of the Prime Meridian along a parallel of latitude. This distance is measured in degrees. When we say that the Longitude of Delhi is 77° East, we mean that Delhi lies East of the Prime Meridian and its distance from the Prime Meridian is 77° The Longitude of a place can be determined by comparing its Local Time with Greenwich Time. Greenwich Time can be known either by radio or by means of a watch which keeps Greenwich Time. Captains of ships generally use a Chronometer for this purpose. A Chronometer is a time piece which keeps accurate time and which is not affected by change in temperature. Now suppose that when it is 2 p.m. at Greenwich, another place shows 5 p.m. The difference between the times of two places is 3 hours, that is 180 minutes. Thus the place is =180 / 4 = 45° away from the Prime Meridian and since its time is ahead of Greenwich. Its longitude will be 45° East.
