Once in a Blue Moon: But Why is the Moon called Blue?

Today (August 31st 2012) the full moon is blue, meaning it is the second full moon in a single calendar month, although, as anyone looking at it tonight will attest, it’s really just its usual colour. So why do we call it blue? Can blue moons really exist?

Strangely, the answer is yes. The effect of a blue moon can be caused by smoke or dust pollution in the atmosphere, masking any red or yellow light to leave only blue. A major volcanic eruption can achieve this effect if its smoke and dust particles are slightly wider than the wavelength of red light (0.7 micrometre). The Krakatoa eruption of 1883 caused the moon to appear blue for the following two years, and smaller eruptions, such as Mount St Helens in 1980, cause the same effect on a more localised level. But is this why tonight’s moon is called blue? And, if so, what is the relevance of it being the second full moon in the same calendar month? Well, the actual colour of the moon is a red – or should that be blue – herring as our blue moon has nothing to do with its colour and everything to do with its more sinister nature. Blue comes from the Old English ‘belewe’, which means betrayer. Tonight’s full moon is a betrayer moon.

The earliest recorded English reference to a blue moon came in a 1524 pamphlet vigorously attacking the Christian clergy. A line of the text reads: “If they say the moon is belewe (blue), we must believe it is true”. But why would such a thing ever be in doubt? And, if it is, who decides if the moon is belewe or not?

It all stems back to the Julian calendar and the reasons it was abandoned for the Gregorian calendar we use today. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per annum against the Julian year. This meant that, over time, the date of these astronomical events bore no relation to what was actually happening in nature. This may have passed unremarked, but for its pernicious effect on the calculation of the date for Easter. When our clergy-basher wrote the pamphlet of 1524, this had become a serious problem.

Easter is unlike most other dates in the church calendar in that it has to be calculated afresh every year. In fact, Easter always falls on the first Sunday following the full moon immediately after the spring equinox. This rather convoluted formula was set down by the First Council of Nicaea in AD 325, when it presumably seemed like a good idea. By 1524, since the equinoxes had advanced by 11 minutes each year, the calculation of the equinox was totally out of sync with the actual seasons and, in some years, spring would have passed and the actual spring full moon come and gone before Easter was due.

Such a delay was important to people since Easter marked the end of the Lent fast; the sooner Easter came, the sooner the fast ended. Unlike today, where Lent lasts a fixed 40 days, it appears the medieval Church merely lengthened the time of fasting to accord with the delayed Easter. No wonder it was unpopular with people.

But what about the actual spring moon following the actual vernal equinox (rather than the date of the equinox set by the Julian calendar, which could be a while off)? Well, if the actual spring full moon occurred before the date of the equinox set by the calendar, the church informed the populace that this was not the full moon that marked Easter and that fasting would continue for another month or so until the first Sunday after the next full moon.

People naturally felt betrayed by such a move and blamed the moon itself for arriving too early. Such a moon was therefore called a betrayer or belewe moon. Today, we say blue, although it is now the second full moon and not the first in a month that receives the affix.

Did people really believe that the moon betrayed them? Look again at the line in that pamphlet, “If they say the moon is belewe, we must believe it is true”. I think people knew where the blame lay and, on Wednesday 2 September 1752, the problem surrounding the calculation of Easter was resolved when the Julian calendar was abandoned, coincidentally leading to a “loss” of 10 days. People objected to that too, but that’s a different story!

9 Responses to “Once in a Blue Moon: But Why is the Moon called Blue?”

  1. Qu says:

    I never knew the full history behind the blue moon thanks for this great post.

  2. And thank you too – glad you enjoyed it ~ Mike

  3. Nadine Jones says:

    This was very informative…thank you for sharing the data that concerns blue moons…. great website too.. I read it daily!

  4. Thanks Nadine – kind of you to say and you’re very welcome. Enjoy the blue moon ~ Mike

  5. Wendy says:

    but I thought…..
    “The term “blue moon” traditionally referred to an extra moon in a season: if a season had four full moons (rather than the more common three), then the third of the four moons was known as a blue moon.
    A mistaken definition, that the second full moon in a calendar month is known as a blue moon, became common in parts of the U.S. during the second half of the twentieth century due to a misinterpretation of the Maine Farmers’ Almanac in the March 1946 Sky & Telescope magazine; this was corrected in 1999.[12]
    Since there are on the average 12.37 full moons in a year, a “blue moon” must occur on the average every 2.7 years (7 times in the 19 years of the Metonic cycle), by either definition.”

    also see: http://en.wikipedia.org/wiki/Blue_moon

  6. Hi Wendy

    Yes, you are quite right that the Maine Farmers’ Almanac used the term ‘blue moon’ in the way you described. This was the original use of the term in the 19th and early 20th centuries. However, in an article for the March 1946 Sky and Telescope magazine, James Pruett got their highly convoluted calculation wrong (as you point out, the Almanac blue moons correspond to the third full moon in a quarter of the year when there were four full moons – whereas normally a quarter year has three full moons) and interpreted a blue moon occurring during a year that had 13 moons. In 1980, a radio programme picked up on Pruett’s explanation and further deduced that a blue moon occurred when there were two full moons in a single calendar month. This provided us with the traditional today – which is actually very recent.

    The medieval explanation of a belewe moon does not necessarily relate to there being two full moons in a calendar month but to the date of Easter. No other moon at any other time of the year was called belewe – unlike today.

    It just shows how traditions change and are added to over time.

    Thanks for giving me the opportunity for fleshing out more of the story ~ Mike

  7. Wendy says:

    Hi Mike,

    Thank you for giving a little more clarification.
    I like to read your blog… interesting stuff :o )

    I’ve been trying to come to terms with so many folks getting excited about this ‘blue moon’ and yet, not quite understanding where it actually comes from. But on the other hand, we are experiencing the creation of an urban myth, which in turn, is becoming reality……
    …..but then, surely it’s the intent that gives the power to magical working at this time, so do names/facts etc really matter???

    Keep writing… keep enlightening


  8. Cheers Mike another great blog. Sorry I didn’t mean to offend you by wishing (pre blog read) “A Happy Blue ( belewe) Moon” The spectacle of a large beautiful moon rising is a site to behold at anytime at any place. Anyway “Bright Moon Blessiings” and still have a lovely weekend. From just down the road :)

  9. Hi Russell
    LOL – I didn’t think about that – I wished so many people a Happy Blue Moon yesterday! But I don’t think it was ever the moon that betrayed anyone!
    Thanks for taking a look at my blog again and thank you so much for the support.
    Have a wonderful (blue!) moon weekend ~ Mike