The Earth is not a sphere
Earth is an oblate spheroid. The equatorial radius is about 21 kilometres longer than the polar radius.
Established2 min read
Newton argued that a spinning, yielding Earth must bulge at the equator. Expeditions in the eighteenth century measured a degree of latitude in Lapland and in Peru and found the polar degree longer, which is what a flattened Earth produces.
Satellites now give the numbers directly. In the WGS84 model the equatorial radius is 6,378.137 kilometres and the polar radius is 6,356.752 kilometres. The difference is about 21 kilometres. A person standing at the equator is farther from the centre of the Earth than a person at the pole.
Caveats
- Local mountains and ocean trenches move the surface by kilometres. The 'pear shaped' geoid is a much smaller wobble, of tens of metres, and is not a second surprising shape hiding under the first.
- For a walk to the shops, a sphere is a fine approximation. The bulge matters for orbits, for pendulums, and for the meaning of 'sea level'.
Sources
- 1Petit G., Luzum B. (eds.). IERS Conventions (2010). IERS Technical Note 36, 2010.