The historical prime meridian runs through a telescope established in 1851 by Sir George Airy at the Royal Observatory at Greenwich, England. Pre-GPS techniques actually responsible for the Greenwich shiftīy Stephen Malys, John H. He and his colleagues detailed their findings in the August issue of the Journal of Geodesy (opens in new tab).įollow us, Facebook & Google+. "It was fun coming up with conclusive evidence as to what really happened with the prime meridian, and why," Seidelmann said. "We found that each place had a different past value for their coordinates, probably based on how gravity caused a local deflection of the vertical." "We contacted friends who knew what their coordinates had been to go out with GPS receivers to take a reading to see whether there had been a change," Seidelmann said. With the aid of colleagues around the world, the researchers also found that the problem was not limited to Greenwich. The offset between these two kinds of verticals explains why the prime meridian now runs 334 feet (102 m) east of where it did, Seidelmann said. The verticals this strategy measures do go through the center of the Earth. In 1984, scientists began using satellites to precisely measure latitude and longitude coordinates on Earth's surface. This means the vertical there "did not go through the center of the Earth," Seidelmann told Live Science. Since Earth's mass is not spread out evenly, this means its gravity field is stronger in some places and weaker in others.Īt Greenwich, Earth's gravity field does not pull straight downward. Anything that has mass has a gravity field that pulls objects toward it, and the strength of this field depends on that body's mass. The problem with this strategy is that Earth's gravity field varies in strength over its surface. The act of "determining the vertical," or knowing which way was straight down, in turn depended on watching a basin of liquid mercury - gravity pulled the fluid downward so it was level with the horizon. These astronomical calculations depended on navigators knowing how their instruments might be tilted with relation to the positions of the moon and stars, explained study co-author Ken Seidelmann, an astronomer at the University of Virginia in Charlottesville. The apparent position of the moon and stars depends on where Earth is facing, and since Earth spins on its axis at a regular pace like a clock, knowing where Earth is facing can help navigators deduce their time and longitude. The prime meridian was used to establish Greenwich Mean Time, upon which all other time zones now depend.īefore clocks accurate enough to pinpoint longitude were developed, navigators gazed up at the night sky to determine time. Īfter inventors created timepieces accurate enough to help navigators calculate their longitude, an international conference in 1884 officially established the prime meridian through Greenwich. If navigators can know what time it is at a fixed reference point, such as the prime meridian, the difference between the time at that reference point and the time wherever the navigators are located can help pinpoint the distance of their ships from that fixed location, and thus determine their longitude. A line of longitude can be thought of not just as a marker of space but also of time - for instance, the eastern United States is an hour or more ahead of the western United States. In contrast, the key to calculating longitude is rooted in time. Latitude is relatively easy to calculate, using an instrument such as an astrolabe to measure the altitude of the sun or a charted star over the horizon. Britain once ruled the waves, and so the Royal Observatory at Greenwich near London ultimately became the reference point for longitude. However, the location of the prime meridian, which marks zero degrees longitude, is completely arbitrary - it could be located anywhere. In the case of latitude, the easiest place to start from and set as zero is the equator. In order to define a location in terms of latitude and longitude, one first has to have starting points both for the lines running north to south, known as meridians, and those running east to west, known as parallels.
0 Comments
Leave a Reply. |