A lighthouse is not trying to be seen. Anything bright is seen. A lighthouse is trying to be recognised, which is a harder job, and it does it with a rhythm.

On a chart, every major light has a written description of how it behaves: two flashes, then dark, repeating every ten seconds. One long flash every fifteen. Three quick, a gap, three quick. Mariners call this the light’s character. At night, from a boat, you can’t judge much about a lighthouse by looking. It’s a point, with no size and no distance. But you can count. You start a watch, you see two flashes and a pause, and you check the page: that’s the one on the headland, not the one further down the coast. The light has told you where you are by the shape of its silences.

I find this oddly moving, because it’s such a modest piece of design. The glass lens that concentrates the lamp into a beam was a real feat of engineering. But the thing that makes a lighthouse useful is the decision to switch it off at regular intervals. A steady light says “something is here.” A pattern says “this is here, and not that.”

It also fails gracefully. If you catch only part of the sequence, you can still rule things out. Two flashes in a row can’t be the light that gives one. You don’t need the whole signal, only enough of it to narrow the field. Most identification works that way, in the end, far more than we admit: not a single clear fact, but a rhythm that is hard to confuse with any other.

The next time you’re somewhere dark and a light blinks on the horizon, count it. It’s probably saying its name.