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‘Once in a Blue Moon’ – Interesting facts and misbeliefs about Blue Moons

Updated on November 29, 2016

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.

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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© Copyright 2012-2014, Zsofia Koszegi-Nagy (zsobig)

© 2012 Sophie

Feel free to share your ideas on the topic!

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    • profile image

      Amy D 

      5 years ago

      I was born on a blue moon and I always thought that was something cool and unique. Cool article.

    • whalefeather2 profile image


      7 years ago

      This month it is Aug 31 and is my granddaughters 18th birthday and she is excited about it being a "blue" moon.

    • zsobig profile imageAUTHOR


      7 years ago from United Kingdom

      @philabustah - You don't have to worry, researches have shown that Blue Moons have no negative or positive biological effects on humanity, so I think not just you, but we are all safe :). All we have to do is to enjoy this beauty that nature gives us!

    • philabustah profile image


      7 years ago from Boston, MA.

      excellent! I'm going to buy a six-pack of blue moon and wait until August. Oh, sugar! That's today! I was born in February, so does

      that mean no blue moon in February is a good thing?

    • zsobig profile imageAUTHOR


      7 years ago from United Kingdom

      Thank you very much Seafarer Mama and Film Fanatic and also thanks a lot for voting up and sharing!

      I will surely try to keep up the good work :)

    • Film Fanatic 603 profile image

      Film Fanatic 603 

      7 years ago

      Very interesting to learn the true origin for the phrase once in a blue moon. Very informative hub that left me with no lingering questions. I look forward to reading more of your work.

    • Seafarer Mama profile image

      Karen A Szklany 

      7 years ago from New England

      Oooh....very lovely hub, zsobig! Blue Moons really are special, and I am looking forward to seeing both of this month's full moon with my daughter.

      Voted Up, Up, UP, Up!...and Liked it on fb!

    • zsobig profile imageAUTHOR


      7 years ago from United Kingdom

      Thank you very much, Ruby, I can't wait August either, if the weather will be fine at nights, it will be a beautiful sight again I bet!

    • Ruby H Rose profile image

      Maree Michael Martin 

      7 years ago from Northwest Washington on an Island

      I love writing about moons too! This is a wonderful hub about blue moons in particular. It was what I wanted to name my website, I have decided on Harvest Moon Junction instead. That covers all moon phases in general. It is a work in forever progress at this time. I am so excited about experiencing the two full moons in August! I love your pictures showing off a "blue" moon. Beautiful! Thumbs up and shared for sure!

    • zsobig profile imageAUTHOR


      7 years ago from United Kingdom

      Thank you, I tried to cover the whole topic, I am glad you like it :).

      Thanks a lot for all for sharing/voting up or pinning.

      I will try to keep my hubs' quality high :).

    • profile image


      7 years ago

      What a great hub! Very interesting, especially with regard to blue moons caused by volcanic ash. That would be a sight worth seeing.


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