![]() We must remember that the position of the Prime Meridian is actually rather arbitrary and could theoretically be located anywhere. ![]() ![]() So what are the implications of this apparent inaccuracy, particularly given that it is the location from where every place on Earth is measured and from which all clocks are ultimately set? Fortunately, the answer is none, really. As a result, the vertical line to the stars and therefore the meridian line on the ground were slightly skewed. But because the Earth is not perfectly round and local gravitational forces vary with terrain, the surface of the mercury at Greenwich was not precisely horizontal relative to the centre of the Earth’s mass. Because the Earth isn’t a perfect sphere, it was impossible to simply move the new line over and maintain an accurate coordinate system.This method, however, assumed that the Earth’s gravitational force that created the horizontal surface on the mercury was both uniform and straight down. This has enabled scientists to determine the true vertical direction and in doing so produced a new meridian slightly to the east of the old one. Today, we have the significant advantage of access to the satellite-based Global Positioning System (GPS), which does not rely on the Earth’s varying gravitational force and uses a more accurate method to calculate the centre of the planet’s mass. To find the exact vertical direction (a line pointing at the precise centre of the Earth’s mass) the Observatory’s astronomers first found the exact horizontal direction (at 90° to the vertical) by looking at the surface of a pool of mercury in a basin. These are the brighter stars, whose positions have been observed over long periods of time and can be used as reference points in the sky. In order to determine the precise angle at which to position the line as it ran through the Greenwich Observatory, its creators used early instruments that were aimed vertically at what are called “clock-stars” in the night sky. How did the Victorian astronomers who created the Meridian get their calculations wrong? It comes down to the fact that the Earth is not a perfect sphere. But we now know that the line, a physical representation of which is visited by thousands of tourists every year, should more precisely be 0.001472° (or 102.5 m) further east. ![]() Since the late 19th century, the Greenwich Meridian has been the line at which most maps mark 0° longitude, the starting point for measuring geographical coordinates in an east-west direction. (Even then, they hesitated to refer directly to Greenwich mean time, preferring the locution “Paris Mean Time, retarded by nine minutes twenty-one seconds.”) This decision did not sit well with the French, however, who continued to recognize their own Paris Observatory meridian, a little more than two degrees east of Greenwich, as the starting line for another twenty- seven years, until 1911. They declared the Greenwich meridian the prime meridian of the world. By the 1800s, maps used leading national observatories (e.g., Greenwich observatory in England), a country’s most prominent maritime port or city (e.g., Philadelphia in the United States), or a religious site (e.g., Jerusalem or Saint Petersburg) to determine their 0o longitude marker.In 1884, at the International Meridian Conference held in Washington, D.C., representatives from twenty-six countries voted to make the common practice official.  For example, in the second century B.C., the Canary Islands were bisected by the prime meridian because these islands were believed to be the western extent of the world. Different people and cultures used different primary reference lines throughout history. Today, the prime meridian is located in Greenwich, England, but this primary reference line of longitude has not always been at its current location. The designation of a line of longitude as the prime meridian (0º) is arbitrary, unlike the degrees of the parallels of latitude that have their zero degree line along the equator, an actual physical feature. The location of the prime meridian and the time zones demonstrate that even the most straightforward mathematical and scientific systems of classification and measurement are not immune to political arguments and boundaries.Â
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