When a snowstorm is coming, people in Marshfield often discuss things like, “Is the storm going to hit at high tide?” or “Is it going to be a full moon?” or “Do you think we’ll have to worry about flooding?” But people rarely discuss exactly why a full moon causes these “flood tides.” And, in reality, it has nothing to do with the appearance of the moon and everything to with the Sun-Earth-Moon arrangement.

During a full moon or a new moon, the Sun, the Earth, and the Moon are basically positioned in a straight line. The moon’s gravity and the sun’s gravity are working together in parallel, and this produces extreme tides called spring tides. However, during a quarter moon, the sun’s gravity is working perpendicular to the moon’s gravity; the sun is literally subtracting water from the high tides on either side of Earth. This results in more balanced tides called neap tides.

So when if we ever have a snowstorm this winter, let’s hope it hits during a quarter moon. Otherwise, Brant Rock could be underwater!

January 14 – Spring and Neap Tides (pg410)