Understanding Longitude: How We Measure Our Location on Earth

Longitude tells how far a place is east or west on Earth. People measure it with lines called meridians that run from the North Pole to the South Pole. The line at longitude 0 is the prime meridian. The equator sits naturally halfway between the poles, but the prime meridian does not. That line exists because people agreed on it, not because nature placed it there. In 1884, officials met in Washington, D.C., and chose a path through Greenwich, England. They selected Greenwich because sea maps and time systems already used that place.

Today a GPS device at the old markers in Greenwich does not read longitude 0. The zero reading appears only after a walk of 102 meters farther east. Scientists in the 1800s did not include uneven gravity or Earth's slightly flat shape when they placed the older line. Later satellite work allowed meridians to run as true paths from pole to pole. In the 1980s, specialists adopted a new mark called the International Reference Meridian. This reference is not locked to one piece of ground, because Earth's surface slowly moves. On the opposite side of the world, longitude 180 serves as the basic idea for the international date line. However, local laws still decide where dates change, so the date line bends around political borders.

Latitude and longitude both use degrees, minutes, and seconds. Lines of latitude stay evenly spaced, but meridians do not. They are widest at the equator and come together at the poles. At the equator, one degree of longitude covers about 111 kilometers. Near latitude 60, that distance falls to 56 kilometers, and at the poles it is zero. People have found latitude with simple tools for thousands of years, but longitude needed harder math and better devices. From the 1500s, governments in Europe offered large prizes for a solution to this problem. At sea, the most practical answer was a reliable clock. A sailor noted local noon, when the sun stood highest, and compared it with a clock set to Greenwich time. A difference of one hour meant 15 degrees east or west, because Earth turns 360 degrees in 24 hours. If local noon came when the Greenwich clock showed 2 p.m., the ship was 30 degrees west.

Movement, changing temperature, and salt air made ordinary clocks unreliable on ships. Only after special marine clocks were built in the 18th century could sailors measure longitude well at sea. Very accurate clocks still matter, but they now travel on GPS satellites and sit in ground stations. A GPS satellite carries atomic clocks that keep time within 40 nanoseconds, or 40 billionths of a second. Each satellite sends radio signals marked with that time. Because the signals travel at the speed of light, a receiver can calculate distance from how long the trip takes. NOAA operates 1,800 ground stations that collect these signals without pause. A phone typically finds its position to about 5 meters. After these station records are processed, longitude can be known to a few centimeters, along with latitude and height.

Why was it difficult to measure longitude at sea, and how did clocks help?