is a geographic coordinate that specifies the east‑west position of a point on Earth’s surface. It is measured as an angle from the prime meridian (0° longitude) to the meridian passing through the point, ranging from 0° to 180° east or west.
What started as a theoretical grid designed by ancient Greek astronomers has evolved into a global system that synchronises human activity. Every time you check the time on your phone, look at a digital map, or order a package from overseas, you are utilizing the invisible, powerful infrastructure of meridian longitude.
By international agreement, the Prime Meridian runs through the Royal Observatory in Greenwich, London, England.
Because a degree can span such a large distance, navigators break degrees down into smaller units for precision: = 60 Minutes (') 1 Minute (') = 60 Seconds (") meridian longitude
一个著名的关键事件发生于1884年10月。当年，25个国家（共41名代表）齐聚美国华盛顿特区，召开了举世闻名的“国际子午线会议”。当时的世界，各国（甚至主要城市）都使用自己的本初子午线，如巴黎、柏林、罗马等。这种混乱在火车和电报发展的时代引发了严重问题。
Directly opposite the Prime Meridian is the 180th meridian, or . This line traverses the vast, mostly empty waters of the Pacific Ocean. It serves as the geographic foundation for the International Date Line (IDL) .
Unlike parallel lines of latitude, meridians are not parallel. They are spaced furthest apart at the equator (approximately 111 kilometers or 69 miles) and converge to a single point at both poles. is a geographic coordinate that specifies the east‑west
For the vast majority of people, the WGS 84 (World Geodetic System) longitude provided by their smartphone is all they will ever need. However, for cartographers, geodesists, and astronomers, a crucial distinction remains: the difference between and astronomical longitude .
Airplanes and cargo vessels do not travel in straight lines on flat maps; they travel along "Great Circles" on a spherical Earth. Navigators use specific meridians to plot fuel-efficient paths and ensure aircraft remain safely separated in crowded international airspace. 3. Geofencing and Logistics
One key characteristic is that the physical distance represented by a degree of longitude is not constant. All lines of longitude converge at the poles, meaning the gap between them shrinks as you travel away from the equator. At the equator, one degree spans about 111 kilometers (69 miles), but at 60° north or south, that distance drops to just 56 kilometers (35 miles), and it reaches zero at the poles themselves. Every time you check the time on your
The resolution of the longitude problem is one of the great sagas of scientific history. In the 18th century, the British Parliament’s Longitude Act spurred inventors like John Harrison to develop the marine chronometer. Because longitude is essentially a measurement of time—one hour of time difference equals 15 degrees of rotation—keeping accurate time at sea allowed sailors to compare their local solar time to the time at a fixed reference point. This breakthrough transformed the meridian from a theoretical line into a practical tool for survival.
While calculating latitude has always been relatively easy—sailors simply measured the angle of the Sun or the North Star above the horizon—calculating longitude at sea was one of the greatest scientific challenges in human history.