The prime meridian is the vertical line that marks the zero degree longitude measurement on the globe of Earth. The line intersects the latitude lines that are horizontal. Not every globe has the same prime meridian measurement, but most measure the earth’s prime meridian as a line that passes through England, France, Spain, Togo, and down to Antarctica.

Unlike latitude measurements, any longitude line begins at the North Pole and ends at the South Pole. Latitudinal measurements combined with longitudinal measurements can help one locate a specific place on a map. They are of particular use in air trafficking, and in determining shipping locations.

Longitude measurements like the prime meridian are also a way to evaluate time. Each space between a longitude measurement represents approximately one hour’s time. Hence if one were to travel from the prime meridian to the next longitude line, which advances one 15 degrees, there would be a difference of approximately one hour in terms of measurements by the sun. Time differences may or may not be accounted for depending upon the country.

There has been some debate about where the prime meridian properly belongs. Currently the measurement for the prime meridian is based on a decision in 1884. The decision was greatly influenced by shipping lines that already had marked the prime meridian as such.

All such measurements are arbitrary, and theoretically, the prime meridian could be anywhere one wished on the globe. However, cooperation is needed so that international systems may use a single prime meridian as the source for giving appropriate directions and for determining such things as flight paths or shipping lines.

Some confuse the prime meridian with the international dateline. This line is in fact directly opposite the prime meridian on a globe. The international dateline has a measurement of 180 degrees and passes through approximately the middle of the Pacific Ocean.

Earth is not the only planet with a prime meridian. In fact we have marked a prime meridian on both the moon and on Mars. These lines are again arbitrary, and should we ever make use of these planets, the lines might change according to the needs of the people using them for map making and information.