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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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.
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