Equator - Exact Length of The Equator

Exact Length of The Equator

In two widely used geodetic standards, the Equator is modeled as a circle whose radius is a whole number of metres. In 1976 the IAU standardized this radius as 6,378.140 kilometres (3,963 mi), subsequently refined by the IUGG to 6,378.137 kilometres (3,963 mi) and adopted in WGS-84, though the yet more recent IAU-2000 has retained the old IAU-1976 value. In either case, the length of the Equator is by definition exactly 2π times the given standard, which to the nearest millimeter is 40,075.016686 kilometres (24,901.460897 mi) in WGS-84 and 40,075.035535 kilometres (24,901.472609 mi) in IAU-1976 and IAU-2000. Although millimeter precision can be important up to the scale of a kilometer, it has negligible physical significance at the scale of a geographic feature such as the Equator.

The geographical mile is defined as one arc minute of the Equator, and therefore has different values depending on which standard Equator is used, namely 1,855.3248 metres (6,087.024 ft) or 1,855.3257 metres (6,087.027 ft) for respectively WGS-84 and IAU-2000, a difference of nearly a millimeter.

The earth is modeled commonly as a sphere flattened 0.336% along its axis. This makes the Equator 0.16% longer than a meridian (as a great circle passing through the two poles). The IUGG standard meridian is, to the nearest millimeter, 40,007.862917 kilometres (24,859.733480 mi), one arc minute of which is 1,852.216 metres (6,076.82 ft), explaining the SI standardization of the nautical mile as 1,852 metres (6,076 ft), more than 3 metres (10 ft) short of the geographical mile.

The sea-level surface of the Earth (the geoid) is irregular, so the actual length of the Equator is not so easy to determine. Aviation Week and Space Technology on 9 October 1961 reported that measurements using the Transit IV-A satellite had shown the equatorial "diameter" from longitude 11 deg West to 169 East to be 1000 feet greater than its "diameter" ninety degrees away.

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