Unlike the equator (which is halfway between the Earth’s north and south poles), the prime meridian is an arbitrary line. In 1884, representatives at the International Meridian Conference in Washington, D.C., met to define the meridian that would represent 0 degrees longitude. For its location, the conference chose a line that ran through the telescope at the Royal Observatory in Greenwich, England. At the time, many nautical charts and time zones already used Greenwich as the starting point, so keeping this location made sense. But, if you go to Greenwich with your GPS receiver, you’ll need to walk 102 meters (334 feet) east of the prime meridian markers before your GPS shows 0 degrees longitude. In the 19th century, scientists did not take into account local variations in gravity or the slightly squished shape of the Earth when they determined the location of the prime meridian. Satellite technology, however, allows scientists to more precisely plot meridians so that they are straight lines running north and south, unaffected by local gravity changes. In the 1980s, the International Reference Meridian (IRM) was established as the precise location of 0 degrees longitude. Unlike the prime meridian, the IRM is not a fixed location, but will continue to move as the Earth’s surface shifts.
Lines of longitude, also called meridians, are imaginary lines that divide the Earth. They run north to south from pole to pole, but they measure the distance east or west.
The prime meridian, which runs through Greenwich, England, has a longitude of 0 degrees. It divides the Earth into the eastern and western hemispheres. The antimeridian is on the opposite side of the Earth, at 180 degrees longitude. Though the antimeridian is the basis for the international date line, actual date and time zone boundaries are dependent on local laws. The international date line zigzags around borders near the antimeridian.
Like latitude, longitude is measured in degrees, minutes, and seconds. Although latitude lines are always equally spaced, longitude lines are furthest from each other at the equator and meet at the poles. At the equator, longitude lines are the same distance apart as latitude lines — one degree covers about 111 kilometers (69 miles). But, by 60 degrees north or south, that distance is down to 56 kilometers (35 miles). By 90 degrees north or south (at the poles), it reaches zero.
Navigators and mariners have been able to measure latitude with basic tools for thousands of years. Longitude, however, required more advanced tools and calculations. Starting in the 16th century, European governments began offering huge rewards if anyone could solve “the longitude problem.” Several methods were tried, but the best and simplest way to measure longitude from a ship was with an accurate clock.
A navigator would compare the time at local noon (when the sun is at its highest point in the sky) to an onboard clock that was set to Greenwich Mean Time (the time at the prime meridian). Each hour of difference between local noon and the time in Greenwich equals 15 degrees of longitude. Why? Because the Earth rotates 360 degrees in 24 hours, or 15 degrees per hour. If the sun’s position tells the navigator it’s local noon, and the clock says back in Greenwich, England, it’s 2 p.m., the two-hour difference means the ship’s longitude is 30 degrees west.
But aboard a swaying ship in varying temperatures and salty air, even the most accurate clocks of the age did a poor job of keeping time. It wasn’t until marine chronometers were invented in the 18th century that longitude could be accurately measured at sea.
Accurate clocks are still critical to determining longitude, but now they’re found in GPS satellites and stations. Each GPS satellite is equipped with one or more atomic clocks that provide incredibly precise time measurements, accurate to within 40 nanoseconds (or 40 billionths of a second). The satellites broadcast radio signals with precise timestamps. The radio signals travel at a constant speed (the speed of light), so we can easily calculate the distance between a satellite and GPS receiver if we know precisely how long it took for the signal to travel between them.
On the ground, NOAA’s National Geodetic Survey manages the Continuously Operating Reference Stations Network, which comprises 1,800 stationary, permanently operating GPS stations. These CORS continuously receive GPS radio signals and incorporate that data into the National Spatial Reference System. The GPS position on a smartphone is accurate to within about 5 meters (16 feet), but processed CORS data can provide longitude accurate to within a few centimeters, along with latitude and height positions.