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
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