The prime meridian is the vertical line that marks the 0° 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 it 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 a person locate a specific place on a map. They are of particular use in air trafficking, and in determining shipping locations.
Longitude measurements are also a way to evaluate time. Each space between a longitude measurement represents approximately one hour’s time. If someone was to travel from the prime meridian to the next longitude line, which advances 15°, 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 is based on a decision made in 1884, which was greatly influenced by shipping lines that already had marked the location. All such measurements are arbitrary, and theoretically, the prime meridian could be anywhere on the globe. Cooperation is needed, however, so international systems use a single longitude line as the source for giving appropriate directions and for determining such things as flight paths or shipping lines.
Some people confuse the prime meridian with the international dateline. This line is in fact directly opposite on a globe. The international dateline has a measurement of 180° and passes roughly through the middle of the Pacific Ocean.
Earth is not the only planet with a prime meridian. In fact, scientists have created such a line on both the moon and on Mars. These lines are again arbitrary, and should humans ever make use of these planets, they might change according to the needs of the people using them for map making and information.