Thursday, 8 January 2009

Blue Moon

Like many of our favourite phrases, once in a blue moon has passed into sporting parlance. Indeed it is not rare at all to hear an Alex Ferguson, a Rio Ferdinand, even a Mark Lawrenson speak about a one-off, a highly-improbable event, a San Marino victory as once in a blue moon.

So what is a blue moon?

I've mentioned before that this is a hotly-debated subject - among blue moon enthusiasts. It's so hot and debatable that there are still two opposing beliefs. But both, through their slightly alternative orbits, lead us to a similar definition: a blue moon is an extra full-moon in either a calendar month (let's use the Gregorian calendar to stop our blue moon buffs getting too excited) or bonus full-moon in a tropical year (yule to yule).

So how does that work?

A full-moon appears once a month, at regular intervals - we know that. So if it falls on, say, the 11th January like this weekend, next month it will fall on the 9th. Roughly a month. But if look closer there's a pattern. Apart from February to March - because February was bullied at school and wants to complicate life for us now - the full-moon moves closer (or remains on the same date) to the beginning of the calendar month as the year progresses.

So in 2009 it's March 11th, April 9th, May 9th, June 7th and so on and smiliar. A countdown.

So the full-moon cycle must be a touch less than a calendar month - and is in fact 29.53 days. So the full-moon appears closer and closer to the 1st of the month, until one month when the next full-moon in the series manages to snuggle in before we flick over the calendar. Two full-moons in a calendar month.

How often does a blue moon occur?

This happens roughly once every two and a half years. Calendar months, you see, are an average of 30.5 days: a day longer than a lunar, synodic (full-moon to full-moon) month. So, as I've tried to explain but not very well, each calendar month brings the full-moon nearer its beginning. By a day a month. So every 30 calendar months we've moved our moon so far along there's room for two in a month. 30 months equals two and a half years. Wow, I just read that back and it works!

As Dr Darren Baskill, an astronomer of some note, pointed out to me,
"Calendar months have been tweaked throughout history, resulting in the arbitrary varying month lengths we are familiar with today. So, the occurrence of a Blue Moon - two full Moons in a calendar month - is a totally non-scientific phenomena. Indeed, in the Chinese calendar the months are strictly defined by the lunar cycle, and so it is impossible to have a blue moon in the Chinese calendar."

The other blue moon

The other offering is that a blue moon is the 13th full-moon in a tropical year. The Southwest Texas State University used evidence from the Maine Farmers' Almanac to find this meaning. These books explain how each season has three full-moons, but when a fourth appears - using the maths above - this constitutes a blue moon: "blue" because, some say, the farmers would mark the third moon of the month of four with a blue dot. So it's the third, not the fourth, that's blue. For those intrigued by this story, more details are only a click away.

Believe and understand of this what you will. But one thing a blue moon isn't is blue. Unless...

So a San Marino victory is more once in a bubonic plague.

And Happy Birthday, Elvis.

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