‘Once in a Blue Moon’ – Interesting facts and misbeliefs about Blue Moons
Once in a Blue Moon
I guess everyone knows the expression ‘Once in a Blue Moon ’ and we also know that it means: rarely, seldom or even very-very unlikely. I doubt that all of us know that the expression dates back to the time before Shakespeare (first reference was recorded in 1528). But do you know what a Blue Moon exactly is and how does it appear? Keep on reading to find out some interesting facts about Blue Moons!
Blue Moon in astronomy
Blue Moon in popular usage means a slightly rare lunar event, when the second Full Moon appears in the same, single calendar month. In fact, this is a (mis)interpretation made by the Sky and Telescope magazine in 1946, when the author of an article misinterpreted a page of the 1937 Maine Farmers’ Almanac . Back then, Blue Moon meant the third Full Moon in a season which had four Full Moons. This showed that in a year there were 13 Full Moons instead of the usual 12, so people named this 13th a Blue Moon to keep the calendar on track.
I guess everybody knows that Full Moon is a lunar phase when the Moon is in opposition of the Sun, more precisely, when the Moon is on the opposite side of the Earth from the Sun. The average time interval between two Full Moons is about 29,53 days (‘about’, because the Moon’s orbit around the Earth is not circular and neither is the Earth’s around the Sun), so you can guess how unlikely it is to see two Full Moons in the same calendar month, as the length of an average month is about 30,5 days only. The only month that can have no Full Moon at all is February.
According to calculations, each century has about 41 months that have two Full Moons, so we can confidently say that the expression ‘Once in a Blue Moon’ actually means ‘once in every 2,5 years’.
Frequency of Blue Moons
As you all know, calendar months either have 30 or 31 days, except for February, which even in leap years have only 29 days. If you read back, we stated that Full Moons appear every 29,5 days, so February will never experience a Blue Moon, not even in leap years as it is shorter than 29,5 days in all cases.
Months that have 30 days usually see 145-165 two Full Moons in every 10,000 (!) years, meanwhile months with 31 days see 475-520. So it is clearly more likely to see two Full Moons in months that have 31 days.
Astronomy books on Amazon
Twice in a Blue Moon – Double Blue Moons
We can speak of Double Blue Moons when there are two separate Blue Moons in the same year. This is very rare phenomenon, it appears once every about 19 to 57 years.
Between 1800 and 2100 there was/will be 10 Double Blue Moons altogether: in 1809 (January-March), in 1847 (January-March), in 1866 (January-March), in 1885 (January-March), in 1915 (January-March), in 1961 (January-April), in 1999 (January-March), in 2018 (January-March), in 2037 (January-March) and in 2094 (January-April). As you can see, Double Blue Moons appear mostly in January and March, but never later than January and April.
In a single century, it is most likely to be 4 or 5 Double Full Moons (even if the previous pattern shows 4/3/3 respectively).
The upcoming Blue Moon
The last Blue Moons occurred on these dates: 2nd and 31st of August, 2012 and 1st and 30th September, 2012.
After these, the next ones will appear in July in 2015 (2nd and 31st of July to be exact), twice in 2018 (January and March - therefore this will be a Double Blue Moon like in 2012) and in October in 2020.
Blue Moon as a song
After all - is it possible to literally see a Blue Moon?
Yes, it is. And not only if you use a blue filter in front of your eyes or your camera, but if the powers of nature ‘help’ us in it. Of course the Moon itself will never turn blue, but a phenomenon will make us see it blue.
The reason for literally seeing a blue moon is volcanic ash. There were several cases when a volcano erupted and the ash-clouds in the atmosphere caused the white moonbeams shining through to ‘turn’ blue. In this way blue moons can exist even for years after one eruption, and in daylight, these volcanic ashes can even turn the sun into a lilac-lavender colour. Volcanoes that caused real blue moons for local people so far are the Krakatoa in Indonesia, the El Chichon in Mexico, the Mt. St. Helens in the USA and the Mt. Pinatubo in the Philippines in the past 120-130 years.
Other Blue Moons
But Blue Moon is not only a term or a lunar event. In modern times, these words are used in many aspects: it is also a brewing corporation in Golden, Colorado in the United States of America established in 1995.
Apart from the brewery, resorts, bars, hotels, cafés have the same name or even a song (written by Richard Rodgers and Lorenz Hart) and a book have been published (by Alyson Noel) under the title ‘Blue Moon’.
So I guess we all can agree that the term ‘Blue Moon’ means much more than the lunatic event itself, as this mysterious expression moves everyone’s imagination.
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