xmlns="http://www.w3.org/1999/xhtml"> Blue Moons
The Moon Guide
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The phase of the Moon right now

Phase
 
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Blue Moon: Copyright Tom King/Galaxy Picture Library
Blue Moon – copyright
Tom King
Blue Moons

’It happens once in a blue Moon’. That's the saying, which refers to an event so rare that it is almost impossible. But can the Moon really appear blue, and how rare is it?

There are several definitions of the term ‘blue Moon’, but in fact it is possible for the Moon to appear blue. It normal colour when it is high in the sky is silvery, although if you could measure its colour very precisely you would find that it is slightly more reddish than sunlight, on account of the colour of the rocks of which it is made. Quite often when we see it, however, it looks yellow, orange or red for the same reason that the Sun can appear these colours, when its light passes through a lot of our atmosphere when it is low down, or because of haze or smoke in the air. This is because gas molecules or dust usually filter out the blue light, leaving the yellow or red part of the light. So how is it possible for moonlight to appear blue? This can only happen when a forest fire or volcanic eruption produces particles of such a size that they filter out the red light and allow the blue light through. Under such rare circumstances the Sun will also appear blue.
Blue Moon. Credit Robin Scagell/Galaxy
This purple twilight in Australia in
1983 followed the eruption of El
Chichón and made the Moon
appear blue by comparison

However, volcanic eruptions can cause another effect which may be seen more often. The dust in the upper atmosphere can cause very vivid twilight colours. If the twilight appears purple, the Moon, if it happens to be in the sky at the time, may appear a vivid blue by comparison. The eye does not record colour faithfully, but instead renders colours by comparison with others, unlike the camera, so this type of blue Moon cannot be photographed,

Another modern definition of a blue Moon, which has nothing to do with its actual colour, is that it refers to the second full Moon in a calendar month. This happens because the interval between successive full Moons is 29.5 days, so if one full Moon occurs on the first or second of the month, there may be another full Moon at the end of the month. Such an event happens every two or three years, so it is not particularly rare.

This occurs for Europe, Africa and America in December 2009. The first full Moon takes place on 2 December, and the next one is at 19.13 UT on 31 December. However, this does not apply to any places which are five hours or more ahead of UT so that the full Moon occurs when it is already 1 January 2010. This applies to much of Asia and Australasia. For them, the blue Moon occurs on 30 January 2010. spacer
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Crescent Moon

The Man in the Moon

Blue Moons













 
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