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).

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