Why Does a Lighthouse Light Flash?
For a lighthouse to be effective as an aid to navigation, it not only had to be seen, it also had to be identified as a unique location. This was necessary if ships were to use it to determine their own location and avoid hazards. For example, let's say you are the Captain of a ship in 1868, bound for San Francisco out of Australia. For the last four days and nights you have been in heavy fog and unable to take any sightings to confirm your location. Suddenly, out of the fog you see a light! "Great," you think to yourself, "the California coast at last!" But, exactly which part of the coast is it? Is this the lighthouse at Point Bonita, or Point Reyes, or Santa Cruz, or... For a lighthouse, just being seen wasn't enough.
The need to clearly identify each lighthouse was often solved by a specific pattern of flashes per minute. Although sometimes lighthouses identified themselves by using colored light, most made use of a flash of light, followed by a period of darkness. This pattern was called the lighthouse's "characteristic."
Pigeon Point's flash pattern was one flash every ten seconds. Today, Pigeon Point is unusual because it still produces a flash every ten seconds, the same pattern it started with, more than 125 years ago. Now, instead of a lard oil burner, a 24" automated aero-beacon is used. In the late 1880s, seventeen different characteristics were used by lighthouses for identification.
Some of the other lighthouse characteristics used in the 19th Century on the California coast:
White flash once each minute
Fixed white light
White flash once every five seconds