Once in a Blue Moon

Once in a Blue Moon
Originally uploaded by Kalamakia

Well it seems my postings have become so rare lately, that I post 'Once In a Blue Moon'. Well today is a such day, that we are going to have a Blue Moon.
Blue Moon, has nothing to do with it's colour. It only means, a full moon that occurs for the second time in a month. Considering that the average interval between two full moons is about 29.5 days & a month has about 30.5 days, it's a rare occurrence to have two full moons in a month. But it occurs about 41 times in a century or only occurs once in about 32 months.
The term 'Once in a Blue Moon' has been in use in medieval England from old times & one such occasion was in a 'Treatyse of the Buryall of the Masse' by William Barlow, the Bishop of Chichester, in 1528.
But the usage of this phrase had had different meanings during these long years of use,

• in 16th century: obvious absurdities, where there could be no doubt.
• in 18th century: as equal to 'never'
• in mid 19th century: a fairly infrequent event

There had been times, where the moon really appeared to be blue. This occurs due to light reflected from moon being scattered by dust particles which are about a micron or so in diameter. In these occasions dust particles were ejected to upper atmosphere during large volcano eruptions such as that occurred in 1883 of Krakatoa volcano or large forest fires that occurred in 1953 in western Canada.
But this year Americans have seemed to have celebrated the Blue Moon on 31st of May, 2007. >>> Blue Moon over North America on Thursday. How can that be? How can they have a Blue Moon two month before New Zelander's had one?

Well it also has a logical explanation. This occurs due to the World Time Zones. When the time of the full moon changes with each time zone it falls to the next month. This is illustrated below.

Time Zone	City	Full Moon 1	Full Moon 2	Full Moon 3	Full Moon 4
GMT + 12hrs	Auckland	2 May @ 22:09	1 June @ 13:04	1 July @ 01:49	30 July @ 12:48
GMT + 04hrs	Moscow	2 May @ 14:09	1 June @ 05:04	30 June @ 17:49	30 July @ 04:48
GMT	Greenwich	2 May @ 10:09	1 June @ 01:04	30 June @ 13:49	30 July @ 00:48
GMT - 05hrs	Chicago	2 May @ 05:09	31 May @ 20:04	30 June @ 08:49	29 July @ 19:48

For more details go to The Blue Moon of 2007.

It seems the method used for calculating the blue moon astronomically also had not been the one used earlier. This new definition had come into use after it had been mistakenly interpreted by an amateur astronomer in an article in 1946 in Sky & Telescope magazine on Blue Moons (see What's a Blue Moon?). It had then been adopted by various other newspapers, radio & TV shows giving wide publicity to this misinterpretation.
The correct definition had been a complex one, that uses seasons rather than months to calculate the blue moon. The third full moon occurring in a season with 4 full moons had been considered to be the blue moon (Usually a season only had three full moons). But it had not been so simple either.
The year they used was 'tropical year' running from one winter solstice to the next. The seasons were defined using a method which gives seasons of equal duration (which uses dynamical mean Sun or fictitious mean Sun, which are imaginary bodies & this method is not used now to define seasons). Then also they used some rules which are laid down in Gregorian calendar reform in 1582.
To see a list of blue moons according to this old definition visit Blue Moon - four Full Moons in a season. According to this it seems there is no Blue Moon in 2007. Phew... after all I'm not doing blog posts Once in a Blue Moon.

references:

• The Phrase Finder: Once in a blue moon
• Science @ Nasa: Once in a Blue Moon
• Obliquity: Once in a Blue Moon