For as long as people have sailed at night, they have needed a way to see the coast before they hit it. The lighthouse is one of the oldest answers to that problem, and one of the most elegant.

Fires on the headland

The earliest lighthouses were simply fires. Communities along dangerous coasts would keep a bonfire burning on a hill or a headland, so that sailors could tell where the land was. It was an unreliable system. Fires went out in the rain, wood was expensive, and a careless keeper could leave a ship blind on the worst night of the year.

The most famous ancient lighthouse, the Pharos of Alexandria, was built in the third century before the common era. Accounts describe a tower more than a hundred metres tall, with a fire at the top and, according to some writers, a polished mirror to throw its light out to sea. It stood for well over a thousand years before earthquakes finally brought it down.

Towers on the rocks

For centuries afterwards, most lights stood on land. Building on the rocks themselves, where ships actually came to grief, was extraordinarily difficult. The Eddystone Rocks, off the south coast of England, saw four different lighthouses in less than two hundred years. The first was swept away in a storm, with its builder inside. The second burned down. The third was so well designed that it was eventually taken apart and rebuilt on land as a monument, because the rock beneath it was wearing away.

These towers taught engineers how to build against the sea: interlocking stone blocks, curved walls that let waves run up rather than strike, and foundations cut directly into the rock.

A better kind of light

The light itself improved just as dramatically. Candles gave way to oil lamps, and oil lamps to mirrors that gathered and focused the flame. The great leap came in the eighteen twenties, when the French engineer Augustin Fresnel designed a lens made of concentric rings of glass. It could bend almost all of a lamp’s light into a single bright beam, visible for many kilometres. Fresnel lenses are still in use today, and the same idea is found in car headlights and overhead projectors.

Each lighthouse was also given its own character: a pattern of flashes, or a colour, so that a sailor could tell not just that there was land ahead, but exactly which piece of land it was.

The end of the keepers

For most of their history, lighthouses needed people. Keepers trimmed wicks, wound clockwork, polished lenses and kept watch through the night, often for weeks at a time on isolated rocks. Through the twentieth century, electricity and automation slowly made the job unnecessary, and most lights now run themselves, checked by occasional visits from maintenance crews.

Satellite navigation has made them less essential still. But on a dark night, at sea, a steady flash on the horizon is still one of the most reassuring things a sailor can see.