I got involved in a discussion the other day, initiated by the science writer Natalie Wolchover, about the geometry of the Earth and our intuitive understanding of it.

There are some interesting points in the replies. For me it definitely comes down to the fact that I have no intuition at all about the radius of the Earth, but I have some about its circumference. The diagram in the tweet above doesn’t help, because it makes the change in radius look very significant, without indicating that on that scale, 6.28m (2π) would be a big change in circumference too.

A lot happens as you head down toward the centre of the Earth of course. We have, as they say, barely scratched the surface. The middle is a very hostile place – with a temperature likely to be similar to the surface of the Sun. The counter-intuitively large distance between us and the centre is of course one reason why this doesn’t melt us, but not the most important one, as I wrote here in BBC Science Focus in response to a question from Daniel Jeffrey.

The picture above, from the St. Anne’s Head on the Pembrokeshire coast, at least helps my intuition that things get pretty heated down there.