Blue moon? Is is time?

August 31, 2012  •  Leave a Comment

Friday 31st August 2012 – and we have a “Blue moon” reported in the press.

But what does a “Blue moon” actually mean?

Is this moon actually blue in colour?

Is tonight’s full moon actually a “Blue moon” in the strictest sense?


Photo 1:

Moon taken very low over the horizon at dawn this morning, a few hours before full "blueness" (of the moon) sets in.



We all (should) know what the phrase “Once in a blue moon” means.

Yep. Something that happens very infrequently – an example might be a hawfinch appearing in one’s garden. (It did once, for me!)


During a “Blue Moon” the colour of the moon changes not a bit (in case any one reading this wondered about that briefly) – it’s still cream in colour.

 A moon with a blue hue would only really arise (quite lidddderally) if the sunlight being reflected back to earth (from the surface of the moon) is refracted by copious amounts of dust thrown into the earth’s atmosphere by a very large volcanic eruption on earth.


So… a blue moon is not blue in hue. That’s the first point to clarify I guess.

But what about the timing of a (not so) blue moon?



Every 29.53 days, our moon undergoes a complete phase cycle (full to new to full).

Our (Gregorian) calendar though is based on our position around the sun not on the lunar cycle.

Since 29.53 days is pretty close to the average days of any of our calendar months, we tend to only have one full moon in each calendar month – but every 2.7 years (regular as clockwork) we have a calendar month with two full moons contained within.

The first full moon in this calendar month has to occur on the 1st or 2nd of the calendar month for a second to occur 29.53 days later (clearly) – and August 2012 provided a first full moon on the 2nd of the month and another full moon tonight – on the 31st of August.


So that’s why many consider this second full moon of August 2012 to be a “Blue Moon”.


But is that strictly true?


Well…. I hate to be a killjoy (and I use this modern definition myself, so I’m shooting myself in the foot with this post really)…. Not really. The second full moon of August 2012 is no blue moon, not if we use the original meaning of the phrase.


Originally, a “Blue Moon” was based on the farmer’s seasonal year. We’re going back many, many, many moons now, but often the farming year ran for 365 days (as now) but from Winter solstice to Winter solstice (December 22nd to December 21st generally), not as we base our calendar now (January 1st to December 31st).

This way of measuring a year was always known as a “Tropical year” (measurement of time).


I’ve mentioned this once (or twice!) before on this website’s “wildlife blog” but even our definition of seasons has changed over time. These days, most of us (even the Met Office) tend to think of the seasons as below…


Winter: 1st December to the last day in February (28th or 29th)

Spring: 1st March to the 30th May.

Summer: 1st June to the 31st August

Autumn: 1st September to the 30th November.


You’ll see this is easy to grasp and based around our Gregorian calendar months – but it’s quite arbitrary really.


The original definition of the seasons was also based on our relative position to the sun, but in a far less arbitrary fashion as below:


Winter: Winter solstice (that day when the sun appears at its (measurable (lowest above the horizon at noon – around December 21st or 22nd in the northern hemisphere) to the “Vernal equinox” (March equinox when very roughly speaking the sun is above the equator exactly and the length of the day is equal to the length of the night – around March 20th or 21st at present for the northern hemisphere).


Spring: Vernal equinox (as described above) to the summer solstice (the day when the sun appears at its highest above the horizon at noon – around June 20th or 21st these days).


Summer: Summer solstice (as described above) to the autumnal equinox (September equinox when very roughly speaking the sun is above the equator exactly and the length of the day is equal to the length of the night – around September 20th or 21st at present for the northern hemisphere). The confusing thing for us these days is that our “midsummer’s day” (June 21st) actually marks the start of summer by the old definition (not the middle!) and I’ll not even go down the road of why the period of time known as “British Summer Time (BST) starts in the fourth week of March in the UK (the start of spring in old money) and ends in late October (a third of the way into Autumn in old money)…


Autumn: Autumn equinox (as described above) to the Winter solstice (as described above).


NB. All the above and dates of full moons are summarised at the end of this blog post in tabular fashion for easier reading!



So…all “tropical years” had four seasons (of course!) and most had twelve full moons – but occasionally there was a (tropical) year with thirteen full moons – meaning one of the seasons had FOUR full moons, not the normal three.


The third full moon in a tropically-measured yearly season which had four full moons in it was referred to as a true “Blue Moon”. (The fourth full moon was not called a blue moon as if it was, the other names for full moons at that time, e.g. “The moon before Yule”, “The Lenten Moon” etc…, would not sit correctly placed in the year).


We really think of a “Blue Moon” these days as the second blue moon in a modern day calendar month (rather than the third in a tropical year season) due only to a mistake in interpretation of old farmers’ almanacs, carried out by an amateur astronomer called James Hugh Pruett just before the second world war. A misinterpretation or a mistake. That’s all.



Of course, times, they have a changed and it’s pretty difficult now to ascertain exactly when a (true) ”Blue Moon” is occurring – rather than the modern definition of a “Blue Moon”.


I’ll use the modern definition of a “Blue Moon” because it’s very easy to work out which moon will be “blue”. The second full moon of August 2012 is “Blue” (tonight’s full moon) and next up will be the second full moon of July 2015 (2.7 years between the two).


Even though this modern definition rankles with the “true blue” conservatives amongst astronomers, we must all remember that the way we measure time is a very human, arbitrary concept and changes relatively frequently – so we should be able to call a moon “Blue” by either definition I think - as long as we appreciate the differences between the two.


It’s rather like us celebrating the end of the old millennium on December 31st 1999 and the start of the (new) millennium on January 1st 2000. Now, if you want to be really pedantic about it, we should have celebrated the start of the new millennium on January 1st 2001 – but where’s the fun in that?


Photo 2:

Moon taken with plane taking off from Heathrow, 2 days before "maximum fullitude" (or "maximum blue-age").



Figure 1.

A comparison of modern-day seasonal dates (general view) and traditional approximate dates for the same seasons.

You can see that these days, we have all our seasons starting and ending three weeks earlier than we traditionally did.





Start date (approx traditional dates)

End date ( approx traditional dates)


1st December

(21st December)

28th/29th February

(20th March)


1st March

(21st March)

31st May

(20th June)


1st June

(21st June)

31st August

(20th September)


1st September

(21st September)

30th November

(20th December)






Figure 2.

A comparison of “modern day Blue moons” (second full moon in a calendar month) and “traditional Blue moons” (third full moon in a tropically-measured season which contains an extra (fourth) full moon).




Modern Blue Moon definition

Traditional Blue Moon definition

March 30th 2010

November 21st 2010

August 31st 2012 (TONIGHT)

August 21st 2013

July 31st  2015

May 21st 2016



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