How Do You Tell One Lighthouse From Another? By How It Blinks

A coast may carry many lights within sight of each other, and a navigator approaching at night needs to know which one is which, since a fix requires identifying the object rather than merely seeing a light. Distinguishing by colour alone gives too few options and colours are hard to judge at distance. The solution is to give each light a distinct rhythm, defined by how long it shows, how long it is dark, and how those intervals repeat, so an observer with a watch counts the pattern and looks it up in a published list. That list, updated continuously, covers every navigational light in the world and gives its position, height, range and pattern.

The published classification uses a small set of defined terms:

• •Fixed, showing a continuous steady light
• •Flashing, dark for longer than it is lit
• •Occulting, lit for longer than it is dark, which is the reverse
• •Isophase, equal periods of light and dark
• •Quick flashing, at a defined rapid rate used for hazards
• •Group patterns, with a stated number of flashes then a longer pause

Choosing between a light that is mostly dark and one that is mostly lit is a practical decision rather than an arbitrary one. A flashing light concentrates its energy into brief intense bursts, which carries further for a given power and is what a major light marking a landfall needs. A light that is mostly on is more visible against a background of shore lights, since a brief flash is easily lost among street lighting and windows while a steady light with brief interruptions stands out, and that is why harbour and channel lights near a town frequently use the mostly-lit pattern. The choice therefore encodes something about where the light is and what it is marking.

Rhythm is one of several properties a navigational light uses, and the others complete the signal. Colour distinguishes the sides of a channel, with a defined convention assigning one colour to each side depending on the direction of travel, and the two major international systems assign them oppositely, which catches out navigators crossing between regions. Sectored lights show different colours in different directions from the same lamp, so a vessel sees white when on a safe bearing and red or green when standing into danger, which is an elegant way of encoding a safe corridor. Height and range are published, since a light appears over the horizon at a predictable distance, which itself gives a position line.

The mechanism has changed completely while the patterns have not. Early lights rotated the whole optical assembly around a fixed lamp, floating enormous lens arrays on a bath of mercury so that a clockwork drive could turn many tonnes with little friction, and the flash occurred as each lens panel swept past the observer. The rotation speed and the number of panels set the pattern. Mercury baths have been removed on safety grounds and replaced by roller bearings. Modern installations frequently use light-emitting diodes switched electronically, which produces any pattern wanted with no moving parts at all, runs on solar power and needs almost no maintenance, which is why lighthouses are now automated and nearly all keepers have gone.

A navigator identifies a light by timing its rhythm rather than by seeing it, and every navigational light in the world has a published pattern, position and range. The classification distinguishes fixed, flashing, occulting, isophase and grouped patterns. A mostly dark pattern concentrates energy and carries further for major lights, while a mostly lit one stands out against shore lighting, so the choice encodes what the light marks.