Why Greenwich?

If you pay attention to navigation, you’ll notice that the town of Greenwich, England (which is located in the metropolitan region of London) keeps coming up. For example, in this section we’ve introduced the prime meridian, which goes through Greenwich. You may also have heard of Greenwich Mean Time, or GMT (also called “universal time (UT),” “universal coordinated time (UTC),” and “Zulu time”), which is defined to be the local time in Greenwich and is now considered the official “world” time. The reason Greenwich has this fame goes back to the fact that the British monarchy built the Royal Observatory in Greenwich in 1675. At the time, one of the great unsolved problems of science and engineering was finding an accurate way for ships at sea to determine their longitude. As a scientific center, the Royal Observatory was assigned a key role in helping to solve this problem. The best solution, which came a few decades later, turned out to be the invention of accurate mechanical clocks, because such clocks allow sailors to determine their longitude by comparing their local time (based on the Sun’s position in their local sky) to the time at some known location — and this “known” location was chosen to be the Royal Observatory. It therefore became customary for sailors to keep track of the time in Greenwich, which therefore became the official “world” (or “universal”) time, and a line passing through the door of the Royal Observatory was later selected as the prime meridian by international agreement.