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Once in a Blue Moon

There has been some quite fervent debate of late as to the colour of yesterday’s full Moon. It did indeed mark the second full Moon of the month, an unusual occurrence, but what does it mean?

First we must understand the lunar cycle. The time observed between each full Moon, or the synodic month as it is known, is roughly 29.5 days. During this time, as the Moon orbits the Earth, we can only see the illuminated portion facing the Sun giving us its phases. It should be clear that when the Moon lies between us and the Sun there will be no visible portion, whereas we would expect to see a full moon when we lie in between.

Usually then, there are 12 full Moons in a year. However, as some quick math will demonstrate (12 x 29.5 = 354), there are some additional days left over in the year. Just like we must occasionally account for leap years (even rarer leap seconds), we find that some years possess 13 full Moons. But which one is the fabled Blue Moon? This is where we meet our conjecture.

Traditionally, all 12 full Moons have seasonal names and correspond either to the ecclesiastical calendar for predicting Lent, or to aid Farmers with milestones for the Seasons. Because each of the full moons are given specific names (eg Harvest Moon), it is important that when choosing the additional moon name it doesn’t put the others out of sync as it is important to retain their correct places relative to each solstice and equinox. This leads to the traditional convention that it is the third full moon in any given tropical season of four full moons that shall be so named Blue.

However, a common misinterpretation of the rule which arose from an easy  mistake is that a Blue Moon is just the second full moon in any given month. Although this is very often the case, there are some exceptions. The Moons are named according to the tropical year (from one Winter solstice to the next) rather than a normal calendar year (1 Jan – 31 Dec). Occasionally two Full Moons may occur in a single month without exceeding that Season’s quota of three – perhaps rarer than a Blue Moon itself!

So, was yesterday a Blue Moon? Although yesterday was a full moon and there are 13 full moons this calendar year, one might notice that the 13th actually occurs after the Winter solstice giving only 12 full moons to the tropical year with each season possessing the standard number of full moons. So, it looks like we will have to wait until next year before we are treated to a true Blue Moon.

For anyone disappointed by this, perhaps they can rest easy knowing that the Moon is actually a little bit blue. The high concentration of titanium oxide and iron in Lunar composition gives the moon a faint blue tint (which can be seen beautifully here).

Pedantry aside though, any opportunity for the public to get excited about astronomy should never be disregarded, and last night (weather permitting) was an excellent opportunity to find a little bit more about our nearest celestial friend. So keep watching the skiiiiis… I mean the skies.


Members of the cast of Grease giving a ‘Blue Moon’

About Sebastian Daniels

Physics with Astrophysics graduate from the University of Manchester with a passion for science enrichment and education, and an interest into scientific representation in popular culture.

One response to “Once in a Blue Moon

  1. crackedaupair ⋅

    But as for why it is called a blue moon whilst not actually appearing blue, wiki seems to suggest that it comes from Christianity. It says Christians have a Lenten moon, which on arriving early was called a ‘belewe’ moon in Old English, meaning betrayer. I’d like to know more really.

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