<P> The first prime meridian was set by Eratosthenes in 276 BCE . This prime meridian was used to provide measurement of the earth, but had many issues because of the lack of latitude measurement . Many years later around the 19th century there was still concerns of the prime meridian . The idea of having one prime meridian came from William Parker Snow, because he realized the confusion of having multiple prime meridian locations . Many of theses geographical locations were traced back to the ancient Greeks, and others were created by several nations . Multiple locations for the geographical meridian meant that there was inconsistency, because each country had their own guidelines for where the prime meridian was located . </P> <P> The term "meridian" comes from the Latin meridies, meaning "midday"; the sun crosses a given meridian midway between the times of sunrise and sunset on that meridian . The same Latin stem gives rise to the terms a.m. (ante meridiem) and p.m. (post meridiem) used to disambiguate hours of the day when utilizing the 12 - hour clock . </P> <P> Toward the ending of the 19th century there were two main locations that were acknowledged as the geographic location of the meridian, France and Britain . These two locations often conflicted and a settlement was reached only after there was an International Meridian Conference held, in which Greenwich was recognized as the 0 ° location . </P> <P> The meridian through Greenwich (inside Greenwich Park), England, called the Prime Meridian, was set at zero degrees of longitude, while other meridians were defined by the angle at the center of the earth between where it and the prime meridian cross the equator . As there are 360 degrees in a circle, the meridian on the opposite side of the earth from Greenwich, the antimeridian, forms the other half of a circle with the one through Greenwich, and is at 180 ° longitude near the International Date Line (with land mass and island deviations for boundary reasons). The meridians from West of Greenwich (0 °) to the antimeridian (180 °) define the Western Hemisphere and the meridians from East of Greenwich (0 °) to the antimeridian (180 °) define the Eastern Hemisphere . Most maps show the lines of longitude . </P>

Explain why lines of longitudes are called meridian of longitude