Thursday, December 31, 2009

Tonight, as the decade ends, we will have the unusual event of a "blue moon" to ring out the old year. The term doesn't actually refer to the moon appearing blue in colour, though this can occur given certain atmospheric conditions. If there is sufficient dust in the air then light of a longer wavelength, ie red, is scattered away from the eyes of the observer, and the light that gets through is shifted to the blue end of the spectrum. this sort of lunar colouration was most prominent after the explosion of Krakatoa in 1883, but it also occurred in the 1950s after large forest fires in Canada and Sweden. It was also reported after the eruption of Mount Pinatubo in the Philippines in 1991, and there were even reports of "blue suns" in that year (see here). I do not recall it being so, and neither do I recall any change in the colour of the Moon after this year's extensive forest fires in BC. I do recall being able to spot the atmospheric haze after the eruption at Mount St. Helen's in 1980. Does anyone else have any memories or references ?

The origin of the phrase "blue moon" event goes as far back as 1528 when it was used in an anti-clerical English pamphlet entitled "Rede Me and Be Not Wrothe" where the author mocks the clergy saying that, if they declared the moon to be blue then they would expect others to believe it. The use of the term to describe a rare event, as in "once in a blue Moon", dates from 1824 (see here). There is some dispute about the origin of the term in relation to an "extra moon". Some say that the Old Farmer's Almanac began to use the term to refer to an extra full moon in a season in the 1800s, while others claim that the Farmers' Almanac didn't begin using the term this way until the 1930s. In any case the usage of the term as referring to two full moons in a month is of even more recent vintage. It's generally accepted that this way of using the phrase began in 1946 when a writer for Sky and Telescope Magazine misinterpreted the Farmers' Almanac rule of an extra moon in a season as being an extra moon in a month. For more on the history of the term see the Wikipedia article on blue moons.

Today's (tonight's) blue Moon takes place at 19:13 UT. Here in the Central time Zone this translates as 13:13. We obviously can't observe the exact instant of the Moon's fullness here in Manitoba at a little after 1:00 in the afternoon. In Australia and East Asia the event doesn't actually occur until tomorrow, January 1, and it is January rather than December that will have a 'blue Moon' there. The actual "blue Moon" will, however, be the second full Moon of January, not the first. The time of an "exact" full Moon is, of course, an instant rather than a night in duration. If you wish to find the exact time of the full moon and to convert it to your own local time consult the Time and Date.Com site where there is a wealth of other information such as sunrise/sunset and moonrise/moonset data. For those who would like a visual representation of the Moon's phases I could suggest the Moon Phase Calender at the Moon Connection site. A "blue Moon" in the sense of a second full Moon in a month occurs about once every 2.72 years.

There is also a partial eclipse of the Moon visible in Australia, Asia, Europe and Africa tonight, though it is not visible in the western hemisphere. This event has already passed , as maximum totality happened at 19;23 UT.

For those interested in matters lunar a couple of years ago Molly featured a series of articles that were a "leisurely cruise" through the various most prominent visible features of the Moon. If you're interested just type the item that you might want to read about (such as 'Mare Criseum', sea of Tranquility, etc..) in the search function of this blog and read all about it. See you in the New Year. It's been an interesting year and an interesting decade.

1 comment:

StarMessage said...

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