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The science and history behind a ‘Blue Moon’ July brings two full moons, a double dose that gives rise to the term "Once in a Blue Moon." Learn more about the origin of the phrase and the effect of full moons.
A full moon rises early Friday behind the 200-foot-tall illuminated Millennium cross on Mount Vodno above Skopje, Macedonia's capital city.Boris Grdanoski / AP
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Friday night brings us the first of two full moons this month. Some almanacs and calendars assert that when two full moons occur within a calendar month, the second full moon is called the “Blue Moon.”  That second full moon will come July 31. The full moon that night will look no different from any other full moon.

On past occasions, usually after forest fires or volcanic eruptions, the moon can indeed take on a bluish or lavender hue.  Soot and ash particles, deposited high in Earth’s atmosphere, can sometimes make the moon appear bluish.  Such a situation was noted across eastern North America in late September 1950, due to smoke from widespread forest fire activity in western Canada.  Also, in the aftermath of the massive eruption of Mount Pinatubo in the Philippines in June 1991 there were reports of blue moons worldwide.

The phrase “Once in a Blue Moon” was first noted in 1824 and refers to occurrences that are uncommon, perhaps even rare. Yet, to have two full moons in the same month is not as uncommon as one might think.  In fact, it occurs, on average, about every 32 months.  And in the year 1999 it actually occurred twice in three months.

For the longest time, no one seemed to have a clue as to where the “Blue Moon Rule” originated.  I myself once suggested that the rule might have evolved out of the fact that the word “belewe” came from the Old English, meaning, “to betray.”  “Perhaps,” I suggested, “the second full moon is ‘belewe’ because it betrays the usual perception of one full moon per month.”

It was not until the year 1999 that the origin of the calendrical term “Blue Moon” was at long last discovered. During the time frame from 1932 through 1957, the Maine Farmers’ Almanac suggested that if one of the four seasons (winter, spring, summer or fall) contained four full moons instead of the usual three, the third should be called a “Blue Moon.”

But thanks to a couple of misinterpretations of this arcane rule, first by a writer in a 1946 issue of Sky & Telescope magazine, and much later, in a syndicated radio program in 1980, it now appears that the second full moon in a month is the one that’s now popularly accepted as the definition of a “Blue Moon.”

Another interesting fact about Friday's full moon is its near-coincidence with the time of perigee — the moon's closest point to Earth.  Perigee occurred Thursday at 7 p.m. ET; the moment of full moon came just over a half a day later at 7:09 a.m. ET on July 2.  At the moment of perigee, the moon’s distance from Earth was 222,107 miles (357,448 kilometers).

This circumstance will result in a dramatically large range of high and low ocean tides during this Independence Day weekend.  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 in this case being derived from the German springen — to “spring up” — and not a reference to the spring season.

While this will be one of the “biggest” full moons of 2004 (the June full moon was slightly closer) the variation of the moon’s size due to its distance is not readily apparent to observers viewing the moon directly.  To those living on the shores near the Bay of Fundy in eastern Canada, however, the 10- to 20-foot (3- to 6-meter) increase in the vertical tidal range makes it obvious when the moon lies near perigee, clear skies or cloudy!