A rare ‘Blue Moon’ is set to light up the sky over Barga this evening. The term refers to the second full moon in a single calendar month. It is only the third Blue Moon in more than two years and the next chance to see one will be in 2015
However, and despite its name, it is not expected to be blue in colour.
According to Nasa: “Most Blue Moons look pale grey and white, indistinguishable from any other Moon you’ve ever seen. Squeezing a second full Moon into a calendar month doesn’t change the physical properties of the Moon itself, so its colour remains the same.”
Fittingly, the full moon will grace the skies on the same day as the private service for Neil Armstrong, the first man on the moon, who died on Saturday 25 in Ohio, aged 82.
But before you go hunt down your old college astronomy textbooks, you won’t find the definition of a blue moon. It truly is fun-filled, accidental folklore – thanks to the Sky & Telescope magazine in the USA.
“This colourful term is actually a calendrical goof that worked its way into the pages of Sky & Telescope back in March 1946, and it spread to the world from there,” says Kelly Beatty, senior contributing editor for Sky & Telescope.
The “Maine rule”: Going back two centuries, the blue moon was defined as the third moon out of four in a given season, according to the Maine Farmers’ Almanacs going back to 1819. (Donald W. Olson, Richard Tresch Fienberg and Roger W. Sinnott, from Sky & Telescope, went searching for the Maine Farmer’s Almanacs to confirm this.)[dw-post-more level=”1″]
Normally there are three full moons in any season, each with a name, such as fruit moon, paschal moon or harvest moon. Since sometimes there are 13 moons in a year, the third moon out of four in a season became the blue moon – a placeholder name — so that the other moons would keep their rightful place in a seasonal order.
Kelly explains that in a 1946 Sky & Telescope magazine article, amateur astronomer James Hugh Pruett (1886–1955) incorrectly assumed how blue moon had been used in the Maine Farmers’ Almanac. Pruett unintentionally mangled the original blue moon definition … and it thus became the second full moon in a given month.
Fast forward to the 1980s, when the newly popular StarDate radio show, with Deborah Byrd, could be heard nationally on public radio stations. There she discussed the second full moon of the month as blue, and popularized information from Pruett’s 1946 Sky & Telescope article. Other media picked up on the story – and an innocuous moon becomes blue and launches into accidental folklore.
People seemed to like the idea of a blue moon, so it stuck. And, although it’s originally a mistake, astronomers seem to have fun with it because interest in astronomy blossoms.
The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and 1951,[10] and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcanoes have also turned the moon blue. People saw blue moons in 1983 after the eruption of the El Chichon volcano in Mexico, and there are reports of blue moons caused by Mount St. Helens in 1980 and Mount Pinatubo in 1991.
On September 23, 1950, several muskeg fires that had been smouldering for several years in Alberta, Canada, suddenly blew up into major—and very smoky—fires. Winds carried the smoke eastward and southward with unusual speed, and the conditions of the fire produced large quantities of oily droplets of just the right size (about 1 micrometre in diameter) to scatter red and yellow light. Wherever the smoke cleared enough so that the sun was visible, it was lavender or blue. Ontario, Canada, and much of the east coast of the United States were affected by the following day, and two days later, observers in Britain reported an indigo sun in smoke-dimmed skies, followed by an equally blue moon that evening.
The key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micrometre)—and no other sizes present. This is rare, but volcanoes sometimes produce such clouds, as do forest fires. Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with a wide range of sizes, with most smaller than 1 micrometre, and they tend to scatter blue light. This kind of cloud makes the moon turn red; thus red moons are far more common than blue moons – source _Wikipedia
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