A 'blue moon,' a rarity in the cosmic calendar, is taking place Friday, July 31, the first time since 2012.
A 'blue moon' occurs when there's a second full moon in one calendar month.
The full moon cycle is 29.5 days so a 'blue moon' is uncommon and has come to mean something rare, hence the expression 'once a blue moon.'
The moon actually won't be colored blue.
Harvard University astronomer Avi Loeb says the moon is far more important to lovers, literature and folklore than to science.
Superstitions around the 'blue moon' include the spookiness of Friday the 13th. This year, the 'blue moon' falls on the 31st, or the reverse of 13. It is also on a Friday.
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