Prime Meridian

latitude and longitude are imaginary lines that divide the surface of the Earth into sections, horizontally and vertically
lines of longitude are known as meridians. Any place has two coordinates: its latitude (north or south of the Equator) and its longitude (east or west of the Prime Meridian)
the Prime Meridian runs through Greenwich, London, UK. It is also known as the Greenwich meridian and it is the basis of the standard time zone system
GMT (Greenwich Mean Time) is a time zone and up to 1972, it was the international time standard. GMT is still used by UK when Daylight Saving is not in use (October to March)
have a look at our digital timepiece Globe Time and check if a certain location is in the 'daytime' part of the Earth or not

Agreeing on a universal Prime Meridian was not an easy affair

Hipparchos was the first astronomer to determine the differences in longitude. Ptolemy, following the Marinus of Tyre, adopted a meridian through the Canary Islands, which marked the western boundary of the world, whereas, to the east, there seemed to be no such boundary.

In 1871 the first International Geographical Congress (IGC) took place at Antwerp. Geographers and scientists of allied disciplines from all nations had tried to fix a common zero for longitude and time reckoning all over the globe.

The view expressed was that for passage charts for all nations, not necessarily coastal or harbour charts, the Greenwich Meridian should be adopted as the common zero for longitude, and that this should become obligatory within fifteen years.

It was also recommended that, whenever ships exchanged longitudes at sea, they should be based on Greenwich. This did not apply to land maps and coastal charts, these should keep its own prime meridian.

However, the 2nd IGC in Rome in 1875 discussed the whole matter again without coming to any further conclusions. France said that if the British were to accept the metric system, then they would accept the Greenwich Meridian. Eventually, it was agreed internationally that a Prime Meridian was needed, and that it should be Greenwich.

Read on about the 1884 International Meridian Conference and its decisions.