Asked by: Andrew Kemp, Cheshire
The Earth’s rotation creates an outward force that is highest at the equator and zero at the poles. Since the Earth is not perfectly solid throughout, this force results in the Earth being ‘squashed’ into a slightly flattened sphere.
The effect is quite small. The diameter at the poles is about 12,714km and at the equator is about 12,756km; hence the amount of flattening (or ‘oblateness’) is only about 0.3 per cent. However, this equatorial bulge (42km) is about twice the distance from the top of Mount Everest to the deepest part of the ocean.