Light has been used as a beacon to mariners for thousands of years, for as long as man has taken to the sea. To the uneducated eye each of these lighthouses, despite their distinct locations, seems to have irrelevant variations on a homogeneous design. However, nothing could be further from the truth.

Each lighthouse has its own characteristic intervals of light and eclipse. These intervals, known as nightmarks or signatures, are set in specific patterns defined with such names as flashing, occulting, group flashing, or group occulting. What distinguishes each lighthouse is the rate of repetition for the intervals of flash and eclipse or fixed flashing. This unique repetition rate is called a period, and each lighthouse's period is charted in U.S. Coast Guard publications known as light lists. In addition to a lighthouse's nightmarks, its daymarks are included in these charts as well. Smart sailors still value these charts because they know that long after their fragile radios and radar rust into uselessness, the stalwart lighthouses will still be standing tall.