However, the tides are the main factor causing the seven-year stretches of erosion. Tides are caused by the position of the moon, the sun, and earth and the distance between the planets. First, there are “spring” tides that occur every 14 and a half days, when the sun the moon and the earth are all lined up in a row as they are during a new or full moon. (The term spring tide comes for the Dutch word “springen” to jump and has nothing to do with the season.) Second, there are perigean tides that occur every 28 days, when the sun, moon, and the earth are lined up and the moon is closest to the earth. Third, there are proxigean tides that occur about every 16 months when the sun, the moon, and earth are all lined up with the earth closest to the sun. Finally, you have extreme proxigean spring tides every 31 years, when all of the elements are combined: the moon is closest to the earth and the earth is closest to the sun, and all are in a line.

Image Courtesy of MacDougall's Cape Cod Marine Service

In 1978, Fergus Wood, author of the book Tidal Dynamics, researched 300 years of erosion events extending back to Mayflower pilgrim William Bradford’s description of a hurricane in the 1600s. Wood found that seven-year stretches of erosion occur during the peaks of the proxigean tides. But his discovery of this phenomenon was largely ignored because Fergus overplayed his hand: Although he had pointed out that storms were needed to trigger these erosion events, his book left the impression that proxigean tides were the primary cause of erosion events. 

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