10. What is a blue moon?

There are actually two meanings to the term "blue moon".

1. Originally, the term is thought to have referred to a rare atmospheric condition which made the moon appear to have turned blue (when viewed from the surface of the Earth).
2. More recently, it is said that a blue moon occurs when there are two full moons in one month. This does not occur very often since the cycle occurs over a 27.3 day period, hence the term "once in a blue moon".

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