The Earth, as most people know, is not a perfect sphere. It bulges at the equator, due to its rotation. Usually it is called an oblate spheroid, but I think this is also not quite accurate.

So, what do we mean by the radius of the Earth? Surely it varies depending on where you measure it.

Really, we mean some kind of average radius, a mean to sea level perhaps.

We also know that the Moon is in orbit around the Earth.

Now, actually, this is not true. What makes bodies orbit other bodies is called gravitation (gravity). This force depends on the masses of the objects and the distance between them (actually, its square). The greater the mass, the greater the gravitational attraction. Double the distance and you quarter the force.

In fact, both bodies rotate about a common centre of mass. If the Earth and the Moon had equal mass, the centre would be exactly half way between them, but it isn’t. The Earth is much more massive, so the centre of mass is somewhere inside the Earth, but not at its centre. Complicated, isn’t it?

But we are talking about the mean distance to the Moon.