Greenwich Mean Time

Greenwich, England defines time and place for the whole world.  Time is measured relative to Greenwich Mean Time (GMT). All places have a latitude (the distance North or South of the Equator) and a longitude (the distance East or West of the Greenwich Meridian).

So whether you are flying an aircraft, sailing a ship or just planning to meet up using your GPS remember that is measured from Greenwich, England:

Longitude 0° 0' 0"
Latitude 51° 28' 38"N (North of the Equator)

The Greenwich Meridian, or Prime Meridian as it is normally called, was arbitrarily given the zero longitude. (At the time, in 1884, Britain had a huge colonial Empire.) The Prime Meridian runs from both poles, and through the "primary transit instrument" (the main telescope) at the Royal Observatory in Greenwich.

The Mean Solar Day, based on a 24 hour clock, divides the Earth into different longitudes, which roughly define 25 different time zones throughout the world. The Universal Day begins at mean midnight at Greenwich. The International Date Line runs 180° from the Prime Meridian. Countires make the decision on the time zones they wish to adopt. For example, China has only one time zone for the entire country.

Source: http://greenwich2000.com/