Once in a Blue Moon

People always seem to be ringing Jodrell with astronomy questions, strange sightings or crazy claims. A few weeks ago, someone called asking what defined a blue Moon. My superviser had answered the call but didn't know the answer without the aid of Google. When he came back to the tea room and told us the question, I was surprised that nobody else in the tea room knew the answer. However it seems that I have been using an incorrect definition anyway.

I had thought it referred to a second full moon in a calendar month. As the average time between full moons is 29.5 days and the average calendar month is 30.4 days, once in a while there will be two full Moons in a month. However, it seems that this definition was mistakenly published by the distinguished Sky and Telescope in 1946. It seems that the original definition comes from the Maine Farmers' Almanac which states that it is the third full Moon in a season with four full Moons. That isn't quite the end of the story though as the Almanac uses the dynamical mean Sun - rather than the actual position of the Sun - which gives the seasons an equal length. So, using the 1946 Sky and Telescope definition, there was a blue moon at the end of July - probably what prompted the caller to ring - but using the Almanac method, there won't be one until August 19th 2005.

Incidentally, although the phrase 'blue Moon' doesn't refer to the actual colour of the Moon, the Moon did appear blue and even green after the Indonesian volcano, Krakatoa, exploded in 1883.

Posted in astro blog by Stuart on Saturday 16th Oct 2004 (02:30 UTC) | Permalink
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