Dr Voge is travelling by ship from south of the equator to the north.

He has a nice little second class cabin which has a bathroom, but no window.

Dr Voge has no compass or any other scientific instruments.

Yet, without leaving his room or talking with anyone, Dr Voge knew exactly when the ship crossed the equator.

We all know how clever he is – but how did he know?

Answers on a postcard…