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The zero meridian used by satellite navigation systems (on the WGS84 datum) is 102.5 metres (336.3 feet) to the east of the line marked at Greenwich.[1] The plane of this geodetic meridian passes through the centre of the Earth, unlike the plane of the astronomical meridian which contains the direction of gravity (indicated by a plumb line) which points opposite to the direction of the zenith, to which astronomical instruments are aligned. The angle between these two meridian planes at the Royal Observatory, the east-west component of vertical deflection, is 5.31″. The WGS84 datum is an average of the various continental drifts. As a result, the astronomical meridian between the vertical crosshairs of Airy's transit telescope drifts toward the east as it is carried by the European portion of the Eurasian tectonic plate, closer to the geodetic meridian, by about one centimetre per year. The zero meridian used by the Ordnance Survey (OSGB36 datum) is about six metres to the west of the line marked at Greenwich. This was the standard meridian before 1851, and the Ordnance Survey simply continued to use it. Universal Time is notionally based on the WGS84 meridian. However, the standard international time UTC can differ from the mean observed time on the meridian by up to about one second (equivalent to about 280 metres at Greenwich), because of changes in the earth's rotation. Leap seconds are inserted periodically to keep UTC in sync with the earth.
One degree of longitude = (111.320 + 0.373sin²φ)cosφ km, where φ is latitude).[2]
Other reference meridiansThe meridian through Greenwich was selected as the Prime Meridian because over two-thirds of all ships and tonnage already used it as the reference meridian on their maps in 1884. Other reference meridians used previously include:
Other planetary bodiesThe prime meridians of the following bodies in the Solar System have been defined:
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