Geographic coordinates are a system of imaginary lines drawn on the Earth’s surface used for navigation and location. This global grid consists of horizontal lines of latitude and vertical lines of longitude. Lines of longitude measure a position’s distance east or west around the planet. These north-south lines have a specific alternative name that is fundamental to cartography and global timekeeping.
The Official Alternative Terminology
The alternative name for a line of longitude is a “meridian.” Meridians are imaginary lines that extend from the North Pole to the South Pole, connecting points of equal longitudinal value. The term originates from the Latin word meridies, meaning “midday” or “noon.”
This etymology reflects an ancient method of timekeeping. Historically, places along the same meridian experienced local noon—the moment the sun reached its highest point—at the same time. The concepts of ante meridiem (a.m.) and post meridiem (p.m.) are derived directly from this geographical concept.
Unique Features of Longitude Measurement
All meridians are halves of “great circles,” meaning they bisect the Earth into two equal hemispheres. Unlike lines of latitude, meridians are not parallel; they converge and meet at both the North and South Poles. The distance between meridians is greatest at the Equator, approximately 69 miles per degree, and shrinks to zero at the poles.
The starting point for the system is the Prime Meridian, designated as 0 degrees longitude. This line passes through the Royal Observatory in Greenwich, England, a location chosen by an international conference in 1884. All other meridians are measured from this baseline, extending 180 degrees to the east and 180 degrees to the west. The 180-degree meridian is the line directly opposite the Prime Meridian.
Practical Application: Connecting Longitude to Time
The primary practical application of the meridian system is determining global time zones. Since the Earth rotates 360 degrees in 24 hours, the planet turns 15 degrees of longitude every hour (360/24).
This calculation forms the basis for time zones, with each zone ideally spanning 15 degrees of longitude and representing an hour’s difference. Traveling eastward across 15 degrees means moving the clock forward by one hour, while traveling westward means setting it back.
The 180-degree meridian is known as the International Date Line (IDL), the point where the date officially changes. It is the anti-meridian to the Prime Meridian, located halfway around the world. Although the IDL generally follows the 180-degree line, it features deviations to accommodate political and geographical boundaries, preventing neighboring areas from experiencing different calendar days.