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Science & Space

Once in a blue moon

By Joe Rao

An image of how the moon might appear when a bluish cast is caused by soot or particles in the air.
An image of how the moon might appear when a bluish cast is caused by soot or particles in the air.
• Full moon feverexternal link
• Skywatcher's guideexternal link

(SPACE.comexternal link) -- Some almanacs and calendars assert when two full moons occur within a calendar month, that the second full moon is called the "Blue Moon."

That second full moon this month will come on Saturday, July 31st, and will look no different than 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 the 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. It was during the time frame from 1932 through 1957 that 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 1980 syndicated radio program, 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 the full moon of July 2nd is its near coincidence with the time of perigee -- its closest point to the Earth. Perigee occurred on July 1st at 7:00 p.m. EDT; the moment of full moon comes just over a half a day later at 7:09 a.m. EDT on July 2nd. 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 being derived from the German springen -- to "spring up," 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 -6 meter) increase in the vertical tidal range makes it obvious when the moon lies near perigee, clear skies or cloudy.

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