Saturday, May 26, 2007

This May is rather unique in that, at least in North America, it will have both a "red Moon" and a "blue Moon". Now these terms are rather deceptive, especially the latter. The Moon may indeed turn a reddish hue when its light is considerably dimmed say during an eclipse or when it is quite low on the horizon. The latter is the case for this year as the Moon is approaching its most southerly point in an 18.6 year long wobble above and below the plane of the ecliptic. The furthest south point of this periodicity will occur along with the full Moon of June 29/30 in North America. At that point the Moon will be a full 5 degrees below the ecliptic and will not even rise at all for points north of 61 degrees latitude. When the Moon is closest to the horizon it may take on a more orange or reddish hue, and the Moon is never closer to the horizon than when it is at the most southerly point of its cycle. The point where the Moon reverses its journey is called a "major lunar standstill", in analogy to the solar solstices. A more lengthy description of the Moon's cycles is provided at .
Then there is the "blue Moon". This is a term that is actually quite modern in origin. The original use of the term was coined by the Old Farmers' Almanac in 1932. The OFA suggested that when any of the four seasons contained 4 full moons instead of 3 that the 3rd should be referred to as a "blue Moon". This became misinterpreted, and the present definition is that when any month has two full moons then the second one is referred to as a "blue Moon". The blue Moon will occur on the full Moon of May 31st in North and South America, but in Australia, Asia, Africa and Europe this moon will actually occur on June 1st. This means that the old world blue moon will be on June 30th this year. Blue moons occur on the average every 32 months, but they can be more or less frequent. In 1999, for instance, there were two blue moons in the course of only three months.
This expression "once in a blue Moon" is meant to convey the idea of "rarity", even though blue moons are not that uncommon at all. The Moon can also appear to take on a blueish tinge if its light is extremely dim. This happened to the unilluminated part of the waxing crescent Moon during the recent conjunction of Venus and the Moon earlier this month, and Molly has certainly observed it at other times during crescent moons. Large amounts of soot and ash in the atmosphere can also make the Moon appear to be blue. The most recent event that led to widespread reports of "blue moons" (and even "blue suns") was the 1991 eruption of Mount Pinatubo in 1991.

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