Blue Moon

Posted: August 1, 2012 in Astrobites, Moon

It’s August 2nd and there’s a full moon in the sky!

That’s an old picture; unfortunately it’s raining so I can’t see the moon right now.  Fortunately, this month we get two full moons, instead of the usual one!

The time between two successive full moons is about 29.5 days – give or take a little.  This means that in most calendar months there can only be one full moon.  However, if a full moon falls within the first day or two of the month then it’s possible for another full moon to fall before the end of the same month.

That’s exactly what’s happening in August; there will be full moons on August 2nd and 31st.  The second full moon on the 31st is called a blue moon…

In the 19th century the Maine Farmer’s Almanac divided the year into quarters and listed the dates of the full moons.  Obviously, the full moon provided much needed illumination for farmers after sunset so they were important.  Normally there’d be three full moons in each quarter of the year, but occasionally there is an extra full moon would fall in a quarter.  Farmers always called the final moon of the quarter the “late moon” and so the third full moon was deemed to be the extra one; they called it a blue moon.  In many cultures the usual pattern of monthly full moons had names like “Grain moon” (August) or “Harvest moon” (September).  The extra full moon which infrequently appeared in the calendar was known as a “betrayer” moon or a blue moon.  The phrase “once in a blue moon”…well, perhaps it’s from this phenomenon.  They aren’t actually that rare; blue moons will occur once every two and half years on average.

One final note; the term blue moon doesn’t refer to the actual colour of the moon.  The moon usually looks white, yellow or orange-red depending on how high it is in the sky (because of how much air the moonlight has to pass through).  However, volcanic eruptions or forest fires can inject just the right size of dust particle into the atmosphere to give the moon (and Sun) a bluish cast.  These kinds of events are also comparatively rare, happening perhaps less often than a blue moon!

  1. trev says:

    Thankyou for that interesting synopsis Adrian. Tommy was really interested but had hoped that the moon was actually going to be blue. Not much hope of a volcanic eruption in the UK though it might be a welcome break from the rain.
    I’d still like to know exactly where the term ‘blue moon’ came from and what it meant?

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