Earth isn't perfectly round, and the factors underlying its shape have undergone a rethink.

The theoretical shape of an entirely fluid, rotating Earth is an ellipsoid with a difference of about 21 kilometres between the equatorial and the polar radius. In reality, however, convective processes in Earth's mantle, triggered by differences in density, account for an additional 100-metre or so difference between the radii.

Frédéric Chambat at the University of Lyon in France and his colleagues recalculated the difference in radii caused by rotation. By subtracting this from the observed value, they found that convection causes a 113-metre difference between the equatorial and the polar radius instead of the 98 metres that is currently used in calculations.

Geophys. J. Int. doi:10.1111/j.1365-246X.2010.04771.x (2010)