The meridian is a virtual line connecting the two poles of the Earth, maintaining the same longitude. Unlike the equator, which can be physically defined, the calculation of meridians for rotating bodies, such as our planet, is completely arbitrary. You can place them where you want, this has been done by people for thousands of years. Longitude, on the other hand, gives east-west coordinates, but only works if you have a “zero point”. This is considered the Prime Meridian and, almost 140 years ago, it was fixed at Greenwich, near London.
The “prime meridian” at Greenwich is mismarked
The states of the world established their “Prime Meridians” for a long time, as they wished, but the crucial moment occurred in England. Here, for centuries, the Prime Meridian passed through St. Paul’s Cathedral in central London until 1721, when it was moved to Greenwich.
Around the same time, the way in which longitude was calculated at sea was developed and the first Nautical Almanac was published by the Royal Observatory in 1767. All these things made Greenwich a place of reference in the field.
“By the end of the 19th century, it all got a bit confusing, because in the meantime the national observatories had been created. You had one in Berlin, one in Pulkovo (Russia), in Rio de Janeiro, in Oslo. The observatories had zero longitude and all the maps of these countries were based on those references,” Louise Devoy, curator at the Royal Observatory in Greenwich, told IFLScience.
“If you were an Englishman trying to do business with Russia, but your map had zero longitude at a different point than the Russians, it became impossible to do that business. Therefore, a large international conference decided in 1884 to decide on a single point of origin valid for all countries as zero longitude.
As, at the time, about 70% of merchant ships used the British reference, it was decided to use the Greenwich meridian in this role. So it was a decision based on pragmatism,” added Devoy.
What is the explanation, what effect does this have on the time zone
But the “prime meridian” at Greenwich is marked wrong, at the present time. Development based on Global Positioning System (GPS) technology showed a discrepancy between where the “Prime Meridian” had originally been established and where the “Zero Meridian” should actually be.
A 2015 study showed that the problem lies with Earth’s gravity. Astronomical methods of determining longitude are based on the precise movement of the stars. But how sure can we be where the “top” is?
The usual approach is to find where the “bottom” is located, and this is done using gravity. After all, gravity pulls everything down to the center of the planet. Or almost.
Gravity is not the same over the entire extent of the planet, because the interior and also the surface of the Earth have multiple variations. Therefore, a line going down to New York is not necessarily parallel to the line going down to Los Angeles.
“The measurements made at Greenwich were affected by local variations in gravity, because the force of attraction is not constant across the globe,” Devoy said.
You can also take the test yourself, provided you are right in the Greenwich Observatory courtyard. If you use your mobile phone’s global positioning system (GPS) to find the prime meridian – the imaginary line from north to south that marks zero degrees longitude – you’ll notice the strange difference.
You will not be on the line drawn in the yard of the observatory, which marks the supposed position of the border between the eastern and western hemispheres, but about 102 meters further east.
The discrepancy between where the prime meridian position is marked and where it actually is was first noticed in the 1960s when satellites were first used to calculate the exact position on the globe.
“After that, the whole system started to change gradually. Therefore, the ‘satellite prime meridian’ is at a distance of about 102.5 meters east of the classical Greenwich meridian,” Devoy concluded.
However, the calculation of astronomical and satellite positions is now done using the International Reference Meridian, which is currently the prime meridian.