moon orbiting the blue planet, 3d illustration
moon orbiting the blue planet, 3d illustration

In August, the full moon will rise twice a single calendar month in New Zealand’snight skies, an astronomical phenomenon commonly known as a Blue Moon.

According to Richard Hall from the Carter Observatory, New Zealand’s NationalObservatory, the Romans are to blame for the extra full moons because theydecided to use a solar calendar, rather than a lunar calendar, meaning that thecalendar months are out of synch with the lunar months.

“Because we now use a solar calendar on occasion, we now get 13 moons in thesame year, instead of 12. Sometimes you get a full moon right at the beginning of themonth, like on the 1st this month, and another one occurring right around the 30th or31st. The second one is called a Blue Moon,

” explains Hall. “If you were using an Islamic calendar or a Jewish calendar,they still use the moon on their calendar and you don’t have any such thing as a Blue Moon.” Where you are on Earth also affects when you are likely to see two full moons in one month. “Our day starts 12 hours inadvance of Greenwich so if the second Blue Moon is right at the end of the month, you might have a Blue Moon in Europebut you don’t have one over here,” says Hall. “Because we are 12 hours in advance it happens on a different day.

“In New Zealand the first full moon in August rose on August 1 and the second full moon is at the end of the month onAugust 30.

But actually, even though “once in a Blue Moon” means that something happens infrequently, in fact a Blue Moon monthcomes around every two to three years. That apparent dissonance between the relative astronomical frequency of a BlueMoon (compared to, for instance, the very occasional Transit of Venus) and its meaning has its roots in folklore, accordingto Canadian folklorist Philip Hiscock.

In an article for the magazine Sky and Telescope, Hiscock explains that term “Blue Moon” is at least 400 years old and itsmeaning has changed many times.

In olden times, Hiscock’s research shows a slightly different meaning of the term: the idea that the moon could be bluewas considered absurd leading to the use of the term as saying something could never happen. Writes Hiscock “Thestatement “I’ll marry you, m’lady, when the Moon is blue!” would not have been taken as a betrothal in the 18th century.

“The latest meaning — a Blue Moon as the second full moon in a calendar month — has only become widespread only inthe last 20 years, Hiscock writes. That meaning apparently came about as a result of a mistake in Sky and Telescope wayback in 1946, when an article called the second full moon in a month a Blue Moon. That meaning then moved into thepopular realm after it was referred to in a radio programmed in 1980.

The real story, according to Sky and Telescope’s “mea culpa” article, is that most tropical years — measured from onewinter solstice to the next — contain 12 full moons and each is named for an activity appropriate to the season. Butoccasionally a tropical year contains 13 full Moons, such that one season has four rather than the usual three. When aseason contains four full Moons, the third is called a Blue Moon, the Sky and Telescope article explains. And these moonsare not the second full moon in a month. The new definition arose, it seems, when tropical and calendar years weremuddled.

And the colour? Well, apparently from time to time the moon has turned blue. NASA’s website tags it to the 1883 eruptionof the Indonesian island of Krakatoa. “The volcano put so much dust in the atmosphere that the Moon actually looked bluein color.

“Philip Hiscock also notes that in 1927, an extra-long dry season when the Indian monsoons were late arriving blew upenough dust for a Blue Moon. And moons in northeastern North America turned blue in 1951 when huge forest fires inwestern Canada threw smoke particles up into the sky.

But as the writers in Sky and Telescope suggest it is probably best not to fret about whether the full moon is really a BlueMoon or not — just wander outside on a full-moonlit night and enjoy the glow of our celestial neighbors.

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