How do lighthouses actually work?

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I’ve been listening to a podcast where they cover the mystery of the Flannan Island lighthouse and it struck me that i’ve never actually understood how a lighthouse works other than shining a beam of light at the ocean? How do the ships know what do and how to steer through the water?

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The basic idea behind lighthouses was to serve as landmarks. A ship’s captain would have a nautical map that would show where the shore was, where any rocks or other dangers were, and of course where various lighthouses were. Then he’d use the lighthouses to help navigate. Most lighthouses had a distinct signal pattern, like Split Rock Light on Lake Superior is a single white beam that flashed every 5 seconds. Some had alternating green and white, or were flashes every 3 seconds, or a flash at 3 and again at 10, or whatever. And it was also known how far out from the shore the light could be seen. So again, for example, if Split Rock could be seen 25 miles from shore, and there was a major shallow spot 5 miles from shore, the captain could gauge where the ship was in reference to which lights he could see and how far out he was from them, allowing him to avoid dangers in the dark. Modern sonar, radar and GPS have made lighthouses almost obsolete as far as navigational beacons, but they’re still very cool, which is why so many are on historic registries

In the olden days, certain areas of the ocean could be very dangerous. There might be areas where the water is mostly deep, but some rocks stick up near the surface, so if you sail through there, you’ll hit the hidden rocks and the ship will sink. So, these places get put on maps and ships try to avoid them. But when you are sailing hundreds of miles, it can be hard to tell exactly where you are. It might be dark and stormy, and maybe you know that there is a dangerous area ahead, but it might be 5 miles ahead or 50 miles ahead, so you don’t know where to go to avoid it. So, people build lighthouses near the dangerous areas. The lighthouse gets put on the map, so now, when the ship gets near the lighthouse, the ship knows where it is exactly and where to go to avoid the dangerous area.

Basically, ships navigated with maps, and lighthouses were like landmarks saying “you are here”.

They don’t actually “flash”. The light is on continuously. It is surrounded by a cage of lenses that slowly rotates at a constant rate, maybe every 30 seconds. A ship in a particular position only sees the light when a beam passes over it.

If it is a 5-second light, it will have six lenses that make the light come out like the spokes of a wheel. (On misty nights, you can see the beams that aren’t pointing directly at you, too.) The lenses can be unevenly set up so you get a beam maybe at 8 and ten seconds, making a double flash.

Charts have all the time periods, colours and locations marked. A particular pattern will only be used once in a big region, so it can’t be mistaken. If you can see two lights (or know your course and speed and tidal drift since the previous light) you can use the bearings with triangulation to find your own position.

Around the North Sea at least it is common to have sectored lights where there is a white beam on the safe passage and red and green beams to the sides. Stray into those and you know which way you need to turn to get back to safe water.

Others just flash a known pattern so you can identify them. If you can take compass bearings to two lights you can plot your position on a chart.

Which podcast was this by the way?

By the way if you are interested in creepy lighthouse incidents check out the Smalls Lighthouse Tragedy.

During the daytime, navigation around coastlines is relatively simple – you just compare the land you see with your charts and you can pinpoint your location.

Around coastlines this can be very important, as the boundary between the land and sea can be very dangerous, with hidden rocks and reefs that can damage the hull of a ship, and unpredictable currents that will change based on tides and other factors. So by being able to locate yourself on your charts, you can make sure to avoid the dangerous bits.

At night however, or when caught in a bad storm, you lose this point of reference – you may not be able to see exactly where the land is, or in enough detail to locate yourself accurately, which puts you in a dangerous position.

The lighthouse is a solution to this – a tall tower with a flashing light at the top means it will be visible from long distances, even in pretty poor weather, so being able to spot a lighthouse means you have something to align yourself with on your charts.
In addition to this, you can also use the flashing light to send a message – by flashing using different speeds or patterns, you can make it so that every lighthouse is different, which means a sailor can compare the pattern they see against the list of different lighthouses and pinpoint where they are.

A lot of this is pretty superfluous nowadays – modern technology like GPS means you always know where you are and what is around you, but jump back a few hundred years to a boat coming into sight of land after crossing the Atlantic, and spotting a lighthouse would let you pinpoint your location on a currently unknown coastline, and allow you to find port.