<P> In two widely used geodetic standards, the Equator is modelled as a circle whose radius is a whole number of metres . In 1976 the IAU chose a radius of 6,378.140 kilometres (3,963.192 mi), later refined by the IUGG to 6,378.137 kilometres (3,963.191 mi) and adopted in WGS - 84, though the yet more recent IAU - 2000 has retained the old IAU - 1976 value . If it were really circular the length of the Equator would be exactly 2π times the radius, namely 40,075.036 kilometres (24,901.473 mi). However, due to the effect of tidal force on the Earth's surface the actual circumference at the equator is 40,075.16 kilometres (24,901.55 mi). </P> <P> The geographical mile is defined as one arc minute of the Equator, so it has different values depending on which radius is assumed, e.g., 1,855.3248 metres (6,087.024 ft) for WGS84 or 1,855.3257 metres (6,087.027 ft) for IAU - 2000 . </P> <P> The earth is modelled commonly as a sphere flattened 0.336% along its axis . This makes the Equator 0.16% longer than a meridian (a great circle passing through the two poles). The IUGG standard meridian is, to the nearest millimetre, 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 (9.8 ft) less than the geographical mile . </P> <P> 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 ° West to 169 ° East to be 1,000 feet (300 m) greater than its "diameter" ninety degrees away . </P>

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