From a functional standpoint, lighthouses serve not only to alert mariners to the presence of the shore, but also assist them in lining up their boats for entry into harbor. Many of Michigan’s lighthouses were built in tandem, known as a “range.”. In Grand Haven and Saint Joseph, for example, a taller light sits closer to shore and a shorter light sits closer to the end of the pier. This allows boaters to gauge whether they are approaching at a proper angle in relation to the harbor, much like a pilot uses runway and approach lights to land an airplane. If the lights appear separate, the mariner must change course until the lights are aligned.
Lighthouses also tell mariners which harbor they are entering or passing. Each lantern has a unique color and pattern of illumination. When a light repeatedly illuminates for shorter periods than it is off, it is termed a “flashing light.” When it flashes in bursts of a certain number, it is called a “group flashing light.” When a light repeatedly illuminates for longer than it is off, it is termd an “occulting light.” When the light turns off, or eclipses, in bursts of a certain number, it is termed a “group occulting light.” When rapid flashing lights are interrupted at regular intervals by longer durations of eclipse, it is termd an “interrupted quick light.” When periods of illumination and eclipse are equal, it is termed an “isophase light.”
These unique patterns of light identify the port, harbor, or town at which the light is located. The unique pattern of each lighthouse is published in the US Coast Guard’s annual Light List. Charlevoix’s North Pierhead Light, for example, is identified in the 2012 Light List as “Fl G 2.5s,” meaning “flashing green at 2.5 second intervals.” Old Mission Point Lighthouse is identified as “Fl W 6s,” meaning “flashing white at 6 second intervals.” Thus, ships can determine their location by reference to the Light List. Lighthouses serve a navigational purpose during the day as well.
The same unique shapes, color combinations, and patterns that make lighthouses appealing architecurally also allow mariners to determine their location during the day. For example, seeing one lighthouse to the south that has a half black and half white base and another to the north that is pure white would tell a mariner on Lake Michigan that they are between Ludington and Manistee.