Blue moon

BLUE MOON

A blue moon is an additional full moon that appears in a subdivision of a year.

The phrase has actually nothing to do with the actual color of the moon because the color isn't really blue. The term has traditionally referred to an "extra" full moon, where a year which normally has 12 full moons has 13 instead.

There could be two exact definitions for a blue moon.

1. Seasonal Blue Moon = The third Full Moon in an astronomical season with four Full Moons (versus the usual three).
2. Monthly Blue Moon = The second Full Moon in a month with two Full Moons.

The reason the second definition of Blue Moon exits is down to an error originally made by amateur astronomer James Hugh Pruett (1886–1955). But today, this is rather seen as the second definition, and not a mistake.