spaceSpace and Physics

There's A "Blue Moon" This Friday - But What Does That Mean?

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Caroline Reid

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1403 There's A "Blue Moon" This Friday - But What Does That Mean?
Shadow Of The Tree. KPG_Payless/Shutterstock.

This Friday, there will be a once in a blue moon occurrence. If you step outside, you might find yourself gazing out at a brimming, full moon. Disappointingly, it won't actually be blue; the term is a misnomer. In fact, a blue moon isn't any different to any other full moon. 

In the past, a blue moon was used to mean "rare," but now the name is given to any full moon that is the second to appear in one calendar month. The lunar cycle is around 29 and a half days, and months of the year are between 28 and 31 days long. The probability of a full lunar cycle, from full moon to full moon, occurring precisely during this time window is therefore not very high.


The next blue moon, by this definition, is going to be on January 31, 2018 – around two and a half years from now. There will be another blue moon two months after that on March 31, 2018, after which it will be another two-year wait.

The term "blue moon" has evolved over the last 200 years or so. Initially, a blue moon was based on a complicated combination of circumstances. In one astronomical season, there are normally three full moons and if there was ever a fourth, then that would be the blue moon. However, this all changed when a hobby astronomer, James Hugh Pruett, wrote an article that was published in the magazine Sky and Telescope in 1946. He defined a blue moon as any second full moon in a calendar month. This definition was incorrect, but it was so much easier to understand that the world scrambled to embrace this new definition. 

You might be feeling a little disheartened that the moon won't be blue, but blue moons in terms of color have been seen in the past under very special circumstances. Usually, these moons are reported after a volcanic eruption or even a forest fire, which release plumes of particles that rise into the atmosphere. The particles happen to be the perfect size to scatter away red light, only allowing blue light through the dust cloud. Out of the ashes, a blue moon appears.

You can watch this video from NASA that beautifully illustrates the phenomenon of blue moons.





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