Johannesburg Centre, Astronomical Society of Southern Africa

Letters to the Editors

31st December 1998    

Dear Editors,

ONCE IN A BLUE MOON
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In 'THE SKY THIS MONTH' for January 1999, FULL MOON will occur on 02 January at 03hrs and again on 31 January at 16hrs.

According to 'CALENDRICAL CALCULATIONS' by Dershowitz and Reingold:
'For obscure reasons, when two full moons occur within one Gregorian calendar month, every 2-3 years, the second is termed a 'blue moon'.

I wonder whether anyone can shed any (moon)light on the reason for this.

Incidentally, full moon on the 31st Jan will be associated with a lunar eclipse so the moon will be rather more reddish-brown than blue.

Patrick Moore in 'GUIDE TO THE MOON' describes blue moons of a different type, not subject to particular calendar dates, but as being due to dust particles in the upper atmosphere caused by giant forest fires. These are very rare and were observed in 1940, 1944 and 1950.

Yours faithfully       
Richard Overy    

Thanks for your letter Richard. We'll pass the buck by asking if there are any of our members out there who may be able to answer this interesting question. We will publish any replies received.

The Editors.

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