Blue moon

You printed an article about “blue moons” (30 July), which refers merely to the predictable two full moons in a calendar month and makes no reference at all to the much rarer phenomenon, when, due to dust in the atmosphere, the moon does actually appear blue in colour.

I saw the moon on 26 September, 1950 on my way home from school. I was by the War Memorial opposite the Albert Halls in Stirling. There was lengthy correspondence about it in the pages of your paper in October 2013, from which I learned that the cause then was forest fires in Canada.

The original “From the 
Archive” item (27 September, 1950) makes no mention at all of the possible interpretation of “once in a blue moon” as “two full moons in a calendar month”. I feel it is fairly recent and wonder when it came into use.

Certainly, to a child of seven the sight of a blue moon was definitely a thing of rarity and wonder.

Marina Donald

Tantallon Place

Edinburgh

Perhaps it was, as you suggest, astrologers who decided that the second of two full moons in a calendar month constitutes a “blue moon”.

However, this has absolutely no astronomical significance and is merely an accident of our irregular calendar. The full moon today is unlikely to appear blue. The term “blue moon” has its origin when, very occasionally, dust or smoke high in the atmosphere filter the moon’s light turning in blue.

The same thing can cause the sun to appear blue. In fact, as you reported on 27 September, 1950, both a blue sun and a blue moon were seen from Scotland and the north of England the previous day due to forest fires in Canada.

It is the rarity of this phenomenon that has given rise to the phrase “once in a blue moon”.

Steuart Campbell

Dovecot Loan

Edinburgh