Why? Because the Earth rotates 360 degrees in 24 hours, or 15 degrees per hour. Each hour of difference between local noon and the time in Greenwich equals 15 degrees of longitude. 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.Ī 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). Longitude, however, required more advanced tools and calculations. Navigators and mariners have been able to measure latitude with basic tools for thousands of years. By 90 degrees north or south (at the poles), it reaches zero. But, by 60 degrees north or south, that distance is down to 56 kilometers (35 miles). At the equator, longitude lines are the same distance apart as latitude lines - one degree covers about 111 kilometers (69 miles). Although latitude lines are always equally spaced, longitude lines are furthest from each other at the equator and meet at the poles. Like latitude, longitude is measured in degrees, minutes, and seconds. The international date line zigzags around borders near the antimeridian. Though the antimeridian is the basis for the international date line, actual date and time zone boundaries are dependent on local laws. The antimeridian is on the opposite side of the Earth, at 180 degrees longitude. It divides the Earth into the eastern and western hemispheres. The prime meridian, which runs through Greenwich, England, has a longitude of 0 degrees. They run north to south from pole to pole, but they measure the distance east or west.
Lines of longitude, also called meridians, are imaginary lines that divide the Earth. Unlike the prime meridian, the IRM is not a fixed location, but will continue to move as the Earth’s surface shifts. In the 1980s, the International Reference Meridian (IRM) was established as the precise location of 0 degrees longitude. 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 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. 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. At the time, many nautical charts and time zones already used Greenwich as the starting point, so keeping this location made sense. For its location, the conference chose a line that ran through the telescope at the Royal Observatory in Greenwich, England. In 1884, representatives at the International Meridian Conference in Washington, D.C., met to define the meridian that would represent 0 degrees longitude. Unlike the equator (which is halfway between the Earth’s north and south poles), the prime meridian is an arbitrary line.
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