There were no national or international conventions which set how time should be measured, or when the day would begin and end, or what length an hour might be. Since the late 19th century, the Prime Meridian at Greenwich has served as the reference line for Greenwich Mean Time, or GMT.īefore this, almost every town in the world kept its own local time. The line itself divided the eastern and western hemispheres of the Earth - just as the Equator divides the northern and southern hemispheres. If you stand with one foot on one side and the other on the left, you are perfectly in the middle of east and west, according to the prime meridian line. Every place on Earth was measured in terms of its distance east or west from this line. The line in Greenwich represents the historic Prime Meridian of the World - Longitude 0º. The decision was based on the argument that by naming Greenwich as Longitude 0º, it would be advantageous to the largest number of people. Therefore the Prime Meridian at Greenwich became the centre of world time. The second was that in the late 19th century, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian. The first was the fact that the USA had already chosen Greenwich as the basis for its own national time zone system. There were two main reasons for the choice. Why does the Prime Meridian run through Greenwich? By comparing thousands of observations taken from the same meridian it's possible to build up an accurate map of the sky. Originally published on meridian is a north-south line, selected as the zero reference line for astronomical observations. "In a number of places, they were off by one heck of a lot," Seidelmann said.įollow Nola Taylor Redd on Twitter or Google+. In every place, the researchers found that the longitudes provided were off by some degree. In another memorable case, one Canadian instrument was reportedly located at its Ottawa, Ontario, observatory when in fact it sat on a local farm its foundation was all that remained, he said. In many cases, they found that observatories of the now-defunct country reported false locations of their instruments. "You're sending people out to try to find an instrument that doesn't exist," he said.įinding instruments in eastern European countries that were once part of the Soviet Union was particularly challenging. According to Seidelmann, most of the instruments had disappeared, and the people who once used them were either retired or dead. That task wound up being more challenging than it first appeared. After determining the locations of observatories in a variety of nations, as described in historical references, they sent colleagues around the world out to determine whether the longitudes of the antique instruments were all off by 334 feet. (Image credit: akasunrise0921 via Wikimedia Commons) Disappearing telescopesĪfter determining why the prime meridian had shifted, Seidelmann and his colleagues investigated how longitudes around the world were affected by the change. However, the actual line of zero degrees lies more than 300 feet (90 meters) to the east. The Royal Observatory in Greenwich straddles the historic prime meridian. Seidelmann's research was published in the Journal of Geodesy (opens in new tab). "They chose not to have a discontinuity in time and let the longitudes correct themselves," Seidelmann said. Faced with the decision of whether to shift the longitudes of the Earth or its time zones to match modern measurements, the BHI chose to have its newly applied system shift the map rather than the time zones. Thanks to satellites, the location of Earth's center of mass has been measured with an accuracy of about the size of a dime in 1984, the accuracy was slightly less precise. In 1984, the International Time Bureau (BIH) redefined the location of the prime meridian based on refined measurements of the Earth's center. "Better technology came along and phased out the optical methods," Seidelmann said. To make those measurements, modern scientists incorporate variations in Earth's rotation, atomic clocks, lasers bounced from Earth to the moon and back, and GPS satellites. Rather than using a basin of mercury, modern technology relies on precise measurements of the line running from the crust to the center of the Earth.
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