How Do You Say Exactly Where You Are?

Step 1 of 5

A ball has no corner to start from.

On a flat map you can measure from a corner. A sphere has none, so a starting line has to be chosen: the equator for one direction, and an agreed line for the other.

Step 2 of 5

Latitude is easy to measure.

How high the North Star sits above your horizon is your latitude, near enough. Sailors could work it out with a simple instrument and no clock at all.

Step 3 of 5

Longitude needed a clock.

The Earth turns fifteen degrees an hour, so knowing the time at home and the time where you are gives your longitude. That meant carrying an accurate clock to sea.

Step 4 of 5

Both are angles, so distances vary.

A degree of latitude is the same distance everywhere. A degree of longitude is widest at the equator and shrinks to nothing at the poles, where the lines all meet.

Step 5 of 5

Every satellite fix still ends in those two numbers.

A phone works out where it is from satellite timings, and then reports it in the same two angles sailors were struggling to measure three hundred years ago.

The prize for a workable way of finding longitude at sea went to a clockmaker, not an astronomer — the answer was a clock that kept time on a rolling ship.