The term meridian, also known as a line of longitude, is the name applied to any imaginary line on Earth’s surface, running north-south and passing through the NORTH POLE and the SOUTH POLE, and at a right angle to the EQUATOR. Prime meridian refers to the meridian designated as zero degree longitude, from which all other points of longitude are measured. With regard to a line of latitude, the equator, at Earth’s widest point of circumference and equidistant from the North and South Poles, is a natural choice for being considered prime, that is, zero latitude. But no one meridian can be so determined, and countries established varying systems for setting the time of day. As a result, early world maps lacked a standardized grid of LATITUDE AND LONGITUDE. In 1884, the Washington Meridian Conference was held to determine an international standard and established that the prime meridian passed through the original site of the Royal Observatory in Greenwich, England. As a result, the prime meridian is also known as the Greenwich meridian. A number of terms have been applied to the standard of time based on the prime meridian: Greenwich Meridian Time, Greenwich Mean Time, Greenwich Civil Time, and Greenwich Time.