Greenwich meridian, << GREHN ihch, GREHN ihj, or GRIHN ihj muh RIHD ee uhn, >> is a north-south line that passes through Greenwich, a borough of London, on a map of the earth. This meridian is often called the prime meridian. It has been designated 0° longitude, and all other meridians of longitude are numbered east or west of it. See Longitude.
The Greenwich meridian is also the starting point for the world’s time zones. There are 24 time zones, each with a width of 15° longitude. The Greenwich meridian lies in the middle of a time zone. Moving east of Greenwich, the time becomes one hour later with each time zone entered. Moving to the west, the time becomes one hour earlier with each zone. See Time.
In 1884, an international conference decided that the meridian which passed through the United Kingdom’s Royal Greenwich Observatory would be the world’s prime meridian. The observatory had played a key role in early navigation and in the development of timekeeping methods needed for navigation. In the mid-1900’s, the Royal Greenwich Observatory was moved from Greenwich to Herstmonceux Castle in Sussex to avoid interference from London lights. It was moved to Cambridge in 1990 and closed in 1998. But the original Greenwich site is still the location of the prime meridian.
