How can lighthouses be told apart at night?
Lights can be distinguished by their “characteristic” or nightmark. Their lights may appear to either glow constantly (fixed) or flash at different rates. Here, the rate is every 7 1/2 seconds, though it has varied over the years. Throughout the centuries, mariners have had lists of lights – tables that list the nation’s navigational aids including the lighthouses, their exact positions, and their daymarks and light characteristics.
