Rather than shining in all directions, lighthouses usually produce a directed beam of light that continuously sweeps in a circle. This allows:
- the light to be more concentrated and so appear brighter.
- the flashing to be more obvious than a continuous light in bad viewing conditions.
- the sweep frequency to be distinctive, eliminating possible confusion by identifying itself (e.g. one lighthouse flashes every 3 seconds, while another one a few miles away flashes every 5 seconds).
In poor visibility, such as at night, navigators can tell which direction the lighthouse is from the ship, but it doesn't help them know which direction the ship is relative to the lighthouse.
Were any lighthouses designed to also provide directional information (similar to aircraft VOR navigation)?
Specifically, a second light, say a different colour light located at the top of the lighthouse, briefly flashes every time the main beam faces north.
Navigators could then measure the delay between the two flashes to determine their line of direction from the lighthouse.
For instance, assuming a standard clockwise rotation as seen from above, if the sweep frequency is every 4 seconds and the main beam appears 1.5 seconds after the north flash, the navigator will know that their ship is currently positioned at
(360° × 4s) ÷ 1.5s = 135°, or directly south-east of the lighthouse.
Being able to do this calculation with two lighthouses would allow the ship's position to be accurately determined.
It seems like a useful and obviously better system, but I've never heard of its actually being used.