A lighthouse on the shore indicates where the shore is. I get that.
However, does it indicate anything else?
For example, does the rate at which the light spins indicate anything? Or if the light doesn’t spin at all, does that tell a captain something about the area near the lighthouse?
Do lighthouses use different colors for their lights to indicate different information, and if so, are the colors universal?
How does a captain use lighthouses to steer his ship? For example, if a captain approaches a strange shore and sees a lighthouse at night, does he stop his ship and drop an anchor until it becomes light out, or can he continue into shore by the lighthouse beacon?
Lighthouses have a “signature” so ships can identify which lighthouse they are seeing. Using their charts (which usually identify the signatures?) they can identify where they are and steer accordingly. It’s a navigation aid that basically says “This is where I am, figure out where you are.” They’re needed near rough coastline or where rocks are close to shipping lanes.
The Lighthouse Stevensons by Bella Bathurst is well worth a read. The Stevenson family (as in Robert Louis) were pioneers in lighthouse design and operation.
Her book The Wreckers makes an interesting follow-up
Just be sure your nautical charts (which give the light signature and day marks) are up-to-date. A few years back (decade or two?) most lighthouses had their signatures changed to dramatically shorten the duty cycle, to reduce power consumption.
These signals are included on navigation charts so (at least in theory) there should be no error in misidentifying a lighthouse. However, for the most part, lighthouses have disappeared as navigation aids due to the cost of maintaining them and the difficulty of navigating by lighthouse in poor conditions. Most current lighthouses (in North America, at least) are maintained as historical sites by local or government organizations. For instance, here is the site for Piedras Blancas which is maintained by the Bureau of Land Management. For the most part, Loran and now GPS has been used for primary marine navigation. Of course, near ports and in busy channel openings such into Puget Sound or San Francisco Bay, and especially where fog conditions are likely, there are still both visible and auditory markers to guide ships with limited visibility into the channel.
The US Coast Guard absorbed the US Lighthouse Service when it was formed and still has the ATON (Aids To Navigation) mission. When I was in the CG, they were in the process of replacing the lighthouses with automatic lights and replacing the lightships with automated buoys. Inactive lighthouses are sold off.
There was a pretty neat lighthouse within walking distance of the LORAN station I was stationed at in the Bahamas. It didn’t use any electricity at all (just as well as there wasn’t much electricity on the island anyway). The light itself, well, it used Coleman lantern mantles and there were some cans of Coleman fuel. The light was on a table in the middle of the lighthouse and the lenses rotated around the perimeter of the room. There were 8 sections. 2 adjacent sections had lenses that emitted a beam of light, the others just, I don’t know, were just minor magnification. Its cycle was 2 flashes every 28 seconds (we had plenty of time there to time them out). Between the lenses and the windows on the land side were canvas drapes so as to not bother the islanders with the light beams.
What fascinated me was how they turned the lenses. It was all clockwork. I checked it out as I climbed up and down the spiral staircase to the light room. It didn’t take winding, there were weights on chains like on an old style cuckoo clock, but heavier. Maybe 10 lb weights. And there were several of them. So, pre-dusk, the Lightkeeper would pull the chains to raise the weights, release the brake, fill the lantern, replace the mantle as needed, pump up the lantern, light it, and set it for maximum bright. And in the mornings, he’d turn the lamp off and apply the brake to the clockwork. And there was a couple cans of Brasso and some Blitz cloths there too.
The captain says, “There is a light that’s flashing blue every 3 seconds. It must be the light at Nantucket Shoals. That light is north-northeast of us, so we must be south-southwest of that light.” (now draw a line SSW from the light on the chart. You know you’re somewhere on that line).