The Greenwich Meridian, which is also known as the zero meridian, base meridian or prime meridian, is the base meridian for measuring longitude. The name “Greenwich Meridian” comes from a point in England through which the meridian passes; to be more precise, its base point is the old Royal Greenwich Observatory.
The Greenwich Meridian was adopted as the reference meridian at an international conference which was held in Washington in 1884; it was presided over by the President of the United States and attended by delegations from a total of 25 different countries. At this conference, it was agreed that:
The second resolution was passed with the opposition of Santo Domingo (today known as the Dominican Republic) and with the abstentions of France (whose maps would continue to use the Paris Meridian as the base reference for several more decades) and Brazil.
One time zone corresponds to 15 degrees of longitude (because 360 degrees correspond to 24 hours and 360/24 = 15).
The International Date Line (where one day changes to the next), which crosses the Pacific Ocean, is located directly opposite the Greenwich Meridian. For practical reasons – and to avoid, for example, island nations having territories in different time zones – this line has been pragmatically adapted to geographic circumstance; as a result, it is not straight, but zigzags at various points, as do some other time-zone defining lines which do not coincide with the meridians.