A lighthouse is a message, and the message is its rhythm

Two lights that look the same would be worse than none

What a lighthouse solves isn’t visibility but identification. A mariner offshore at night sees a point of light and needs to know which point of light it is, because a light mistaken for its neighbour puts a vessel on exactly the rocks the tower was built to mark.

The solution is to give each light a character: a repeating pattern of flashes, occultations or fixed periods, with a defined cycle time. One light flashes twice every ten seconds, the next along shows a group of three every fifteen, and the pattern is published on charts and in official lists.

This is why counting is the right way to look at a light from a beach at night. The rhythm is the name, it is timed with a watch by anybody navigating seriously, and it has usually stayed the same for a very long time because changing it would invalidate every chart in circulation.

Colour and sector do the rest of the work

Many lights are sectored, meaning the lantern shows different colours through different bearings. A vessel in the white sector is on a safe approach; drifting into the red or the green tells the watchkeeper immediately which side of the safe channel they have wandered towards.

That single arrangement lets a fixed tower give a moving ship continuous positional information without any equipment beyond an eye and a chart, which is a remarkably economical piece of design. Leading lights do something similar by pairing two towers that align only on the correct approach line.

Buoys and beacons extend the same grammar out into the water, with shapes, topmarks and colours coded to say which side to pass and what the hazard is. The systems are internationally agreed, with one long-standing regional difference in whether red is left to port or to starboard on entering a harbour.

The towers are where they are because of what happened there

A lighthouse marks a hazard that was killing people, and the more remote and difficult the tower, the worse the hazard usually was. Rock lighthouses standing in open water, built in the eighteenth and nineteenth centuries with astonishing difficulty, mark reefs that lie in the path of major trade routes.

The construction techniques are worth knowing about before visiting one. Interlocking dovetailed masonry courses, a curved profile designed to shed the force of a wave, and entrances high above the base are all responses to the fact that the sea arrives at these structures with enormous energy.

Their keepers lived in rotations of weeks with relief by boat or, later, by helicopter, and the accounts of that life are the human part of the story. Automation removed the keepers from nearly every light over the second half of the twentieth century, which is why so many keepers’ cottages are now holiday lets.

Satellite navigation didn’t make them obsolete, quite

The obvious question is why any of this survives when a receiver gives a position to a few metres. The answers given by maritime authorities are consistent: electronic systems can fail, can be jammed or spoofed, and small craft may carry nothing at all, so a visual mark that needs no equipment retains value.

That said, the number of lights has been reduced in many countries, ranges have been shortened, and lamps have been replaced with far smaller and cheaper units. A tower that once carried a rotating optic the size of a room may now be lit by something you could carry in one hand.

The debate about how much to keep is genuine and ongoing. Maintaining an offshore tower is expensive, the shipping that funds it through dues has changed, and different national authorities have reached different conclusions about the right level of provision.

Visiting one, and what to look for

A good proportion of accessible lighthouses are open to visitors, either as museums or through a local trust, and the object usually worth the climb is the optic. A large Fresnel lens is a piece of precision glass built to gather a lamp’s output into a beam, and seeing one closely explains the whole technology.

The mechanism underneath is the other surprise. Rotating optics were commonly floated on a mercury bath to reduce friction and turned by a clockwork drive that a keeper wound by hand at intervals through the night, which is why the job involved staying awake rather than merely being present.

From outside, the daymark matters too. The bands, stripes and colours painted on a tower exist so it can be identified in daylight when the light is not showing, so the pattern is not decorative and, like the flash character, it is recorded on the chart.