Today, being in Dublin, I went on a little day trip to the charming seaside town of Dun Laoghaire (when you say it out loud, it sounds like “Dun Leary” — don’t ask). While there, I had a splendid time exploring the National Maritime Museum of Ireland.
I learned all sorts of things today about sailing, lighthouses, engine technology, nautical charts, shipwrecks, lifeboats, sextants, trade routes, and many other fascinating topics, far too numerous to list here.
But one fact in particular really stuck with me: In their hay day, every lighthouse flashed at a unique rate. That is, the motor that spun the light around was set to a different rate of rotation for every lighthouse.
And the reason for this was simple and ingenious: If your ship was lost out at sea, and all you had to navigate by was the faint pulse of a distant lighthouse, you could time that pulse and you would know where you were.
This concept is also the basis for heterodyning, the core technology behind broadcast radio and television — a more recent innovation whereby multiple signals can be distinguished by their differing carrier frequencies.
What I love about the pulse frequency scheme for identifying lighthouses is the way it shows that people are not getting more clever over time. People were always clever. It’s just that, at various times in history, they have access to different technologies for showing how clever they can be.