Blue moon facts for kids
A blue moon is an additional full moon, where a year which normally has 12 full moons has 13 instead. The "blue moon" reference is applied to the third full moon in a season with four full moons. This happens every two to three years.
One lunation (an average lunar cycle) is 29.53 days. There are about 365.24 days in a year. Therefore, about 12.37 lunations (365.24 days divided by 29.53 days) occur in a year. In the widely used Gregorian calendar, there are 12 months (the word month is derived from moon) in a year, and normally there is one full moon each month. Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles. The extra days accumulate, so every two or three years, there is an extra full moon. The extra full moon necessarily falls in one of the four seasons, giving that season four full moons instead of the usual three, and, hence, a blue moon.
The phrase has nothing to do with the actual color of the moon, although a literal "blue moon" (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions: e.g., if volcanic eruptions or fires leave particles in the atmosphere of just the right size to preferentially scatter red light.
Owing to the rarity of a blue moon, the term "blue moon" is used colloquially to mean a rare event, as in the phrase "once in a blue moon".
Origin of the term
The term "blue moon" arose from folklore, the "blue" replacing the no-longer-understood belewe, 'to betray'. The original meaning was "betrayer moon", referring to a full moon that would "normally" (in years without a blue moon) be the full moon of spring, while in an "blue moon year", it was "traitorous" in the sense that people had to continue fasting for another month in accordance with the season of Lent.
Blue moon Facts for Kids. Kiddle Encyclopedia.