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What is a Blue Moon?

We all know the idiom - if something never happens, or very rarely, it happens once in a blue Moon. But what is a blue Moon, what does the term mean and how often does a blue Moon actually occur?

Blue Moon in the night sky

Today's calendar is rooted in astronomy: a day is based on the rotation of the Earth on its axis, a year comes from the time it takes Earth to orbit the Sun, and a month is based on the revolution of the Moon around Earth.

For the most part, the lunar cycle and our calendar are in sync, the Moon waxing and waning each month, giving rise to the phases of the Moon, with about 29.5 days between full Moons.

Typically, then, we see 12 full Moons a year, one on average falling neatly into each calendar month. But the two calendars don’t precisely match up. A calendar year contains around 11 days more than the number of days in 12 lunar cycles, so eventually the difference makes itself known.

It’s then that we get two full Moons within a calendar month, and this has led to the informal meaning: a blue Moon is the second full Moon in a month. Perhaps surprisingly, the name 'blue Moon' is a rather modern expression, from a calendrical perspective, first noted in a 1946 edition of Sky & Telescope magazine.

Like many expressions, “once in a blue Moon” is not particularly accurate, as they actually occur quite frequently, every two to three years. And the next one is going to be on 22 August.


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