Based on my recent discussion of blues (colours, not moods), I though I should describe what a blue moon is. 'Once in a blue moon' is a common phrase for an uncommon event. However, the moon is never blue (unless there are smoke or dust particles in the atmosphere, then the moon can appear bluish).
I've seen the moon turn a blood red – a frightening event if I didn't know why it changed colour. On that night it was a lunar eclipse. The earth had moved between the moon and sun, casting the moon into its shadow. Normally, the moon reflects sunlight directly from the sun making it a bright feature in the night sky. When the earth is in the way, the only sunlight to reach the moon is refracted around the earth and as a result the moon takes on a blood red colour (at least it did on the night I watched).
A blue moon is a completly different event, more of a bookkeeping phenomenon. A calander month is on average 30.5 days long, while the time between full moons is 29.3 days. Normally, there is one full moon a month but, on a rare occasion, a month can have two full moons. There are different ways of determining which of the full moons is the blue one. Typically, a season has three full moons, however when one of the months has an extra full moon, the third full moon is condidered a blue moon. This is so rare, a blue moon will occur only seven times in 19 years, which works out to one every two to three years. Mark your calenders as the next one will occur on August 21, 2013.
Been reading 'The Field Guide to Natural Phenomena' by Keith Heidorn and Ian Whitelaw – so far it promises lots of interesting tidbits.