You won't see a blue moon like this again until 2018

·Crime Contributor
NASA's view of the blue moon.
NASA's view of the blue moon.

Friday is one of those days that we only have once in a blue moon. 

In fact, this Friday July 31,st  will actually be a blue moon when it turns full at 6:43a.m. ET. That's because it will be the second full moon in the month.

Unfortunately, the rarity of the blue moon isn't quite as rare as it sounds since there is more than one kind of blue moon.

This week's blue moon is all about the rarity of having two full moons in the same month. Usually that happens about once every 2.6 years, although in 1999 it happened twice in three months. The next time skywatchers are likely to see a blue moon is in 2018.

“The terms seems a little convoluted as to why the term blue was used,” The Weather Network meterorologist and science writer Scott Sutherland tells Yahoo Canada News.

“The Farmers' Almanac gives the original seasonal definition as being four full moons in a season rather than three full moons,“ says Sutherland.

“It seems that may have just been following the system for naming all the other moons, but as these come every three years they have tacked on the phrase blue moon. The phrase once in a blue moon may have been around much longer.”

In fact, Yahoo Canada News did a little probing and found the second definition of blue moon (twice in one month) came about in the 1900s when it was misidentified.

The term “once in a blue moon” is believed to have originated in the early 1800s, but wasn’t actually tied to the moon’s phases until sometime later.

The Maine Farmers' Almanac – not to be confused with the Farmers' Almanac (also published in Maine), originally made the suggestion around 1932 that a season with four full moons should be called a Blue Moon.

However, an article in 1946 by amateur astronomer Hugh Pruett (1886-1955) in Sky and Telescope magazine confused the term, leading to a second definition of blue moon – a full moon twice in a month.

The second-full-moon-in-a-month definition came into widespread use when incorrect information from the 1946 article was repeated on the radio program StarDate on January 31, 1980 -- 35 years ago Friday.

Since then, the term has stuck.  

So don't look for the moon to be any of the famous shades of blue from Persian Blue to cobalt or azure, or even Arctic blue. But for those of you holding out for a truly blue coloured moon – it can happen when the atmosphere is full of tiny particles. 

So depending where you live, it might even happen this year. 

Sutherland says the full moon looking blue is because of the science of light refraction.

“It can happen when you add particulate matter into (the atmosphere), if there are really tiny, micron-sized particles, and there is light reflecting off the moon,” says Sutherland.

In fact, NASA says “the key to a blue moon is having in the air lots of particles slightly wider than the wavelength of red light (0.7 micron)--and no other sizes present.”

Sutherland says when that happens, we can get a blue moon.

“Those are the right size to scatter the reds more frequently than the blues, so as that light reflects off the moon at us, the reds are scattered by the particulate matter,” he says.

“That's usually from volcanoes or large eruptions that push matter really high into the sky, or forest fires, really really big ones like we had in Saskatchewan and Alberta may generate that same thing.”

|NASA wrote about blue moons back in 2004, when coincidentally there were also two full moons in July.

There have been times in history when a blue coloured moon was common – back in 1883 with the eruption of Krakatoa which filled the atmosphere with ash clouds that stayed so long that blue moons were prevalent for years.

Even the sun had a lavender hue at times, NASA said in its 2004 article.

So the next time people might be expecting a change in the moon's colour will likely be in September with the harvest moon. But, unlike popular belief, harvest moons aren't always orange.

The Harvest moon, when the full moon is closest to the autumn equinox (This year on September 23) , can sometimes appear slightly bigger and can take on a different hue depending on particles in the atmosphere. And between Sept. 27-28 this year there will be a “supermoon” and the “blood moon eclipse.”

History shows there are plenty of superstitions surrounding the moon and its colours.

* The Romans thought staring at a full moon would make a person go insane – hence the term “lunatic” - a phrase that unfortunately still creates stigma for those with mental health issues or illness.

* Farmers in days of olde would plant their crops according to the phases of the moon to hopefully get a better harvest.

* Passages in the Bible described a Blood Moon as a bad omen, but the moon can often look red because of forest fires and particles in the atmosphere.

There are also plenty of superstitions for every phase of the moon, but that's a different story.

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