Twice each month, the Sun, Earth and Moon form a straight (or nearly straight) line in space, called a “syzygy.” At these times, the Moon is either at full or new phase and we can expect tides to run somewhat higher than normal. Twice each year, once at full and once at new phase, the Moon will also be very near perigee, its closest point to the Earth. These perigee Moons can produce the highest tides of the year.
On August 10th at 2 p.m. EDT, the Moon will arrive at its closest point to the Earth in 2014: a perigee distance of 221,765 miles away. Just nine minutes later, the Moon will officially turn full. Though full Moon theoretically lasts just a moment, the moment is imperceptible to ordinary observation, and for a day or two before and after most will speak of seeing the nearly full Moon as “full” (the shaded strip is so narrow, and changing in apparent width so slowly, that it is hard for the naked eye to tell whether it’s present or on which side it is).
The near coincidence of the August 10th full Moon with perigee will result in a dramatically large range of high and low ocean tides. Any coastal storm at sea around this time will almost certainly aggravate coastal flooding problems. Such an extreme tide is known as a perigean spring tide, the word spring being derived from the German springen — to “spring up,” not a reference to the spring season.
Every month, “spring tides” occur when the Moon is full and new. At these times the Moon and Sun form a line with the Earth, so their tidal effects add together. (The Sun exerts a little less than half the tidal force of the Moon.) “Neap tides,” on the other hand, occur when the Moon is at first and last quarter and works cross-purposes with the Sun. At these times tides are weak.
For those who are well versed in mathematics, tidal force varies as the inverse cube of an objects distance. This month the Moon is 12.2 percent closer at perigee than it was at its extreme apogee – its farthest point from Earth in 2014 – on July 27. Therefore, on August 10th, the Moon will exert about 42 percent more tidal force compared to the spring tides just two weeks earlier.
The 2014 Observer’s Handbook of the Royal Astronomical Society of Canada refers to the August 10th Moon as the “largest full Moon in 2014,” – in recent years, a popular connotation for a full Moon that coincides with perigee has been “Supermoon” – the variation of the Moon’s distance is not readily apparent to observers viewing the Moon directly. To those living near the Bay of Fundy in eastern Canada, however, the rapid increase in the vertical tidal range of as much as 20 feet or more makes it quite obvious when the Moon is near perigee, whether the sky is clear or cloudy!