Once in a Blue Moon

Once in a Blue Moon ... is a common way of saying not very often, but what exactly is a Blue Moon?

According to the popular definition, it is the second Full Moon to occur in a single calendar month.

The average interval between Full Moons is about 29.5 days, whilst the length of an average month is roughly 30.5 days. This makes it very unlikely that any given month will contain two Full Moons, though it does sometimes happen.

On average, there will be 41 months that have two Full Moons in every century, so you could say that once in a Blue Moon actually means once every two-and-a-half years.

Blue Moon Calculator

Is there a Blue Moon this year? Next year? The year I was born?

The Blue Moon Calculator knows the answer!

Frequently Asked Questions (and their answers)

Click the button at the right to see a more detailed answer to each question.

Question Brief answer Long answer
When will the 2012 Blue Moon occur? August or September, depending on your timezone.
Isn't your definition of Blue Moon wrong? Yes and no. It's the definition that's currently popular, but it only dates back to 1946. There's an older, traditional meaning that was used in farmers' almanacs in the 19th century.
How often is there a Blue Moon by the old definition? Once every two or three years, just like the new definition, but not in the same years.
Does a Blue Moon have any effect on human behaviour? It has no more effect on human behaviour than any other Full Moon.
Where did the expression Blue Moon come from? It's been part of the English language for four hundred years. During that time, it's had half a dozen different meanings.
Does the Moon actually change colour during a Blue Moon? No, but there have been rare occasions when the Moon really did turn blue.
Why is there a bright ring around the Moon? It's called a halo, and it's caused by ice crystals high in the Earth's atmosphere.
Which months have the most Blue Moons? As you might expect, the 31-day months are the winners. October, August and July take the top medals.
How often does a year have two Blue Moons? Four or five times in a hundred years.
Can a month have no Full Moon? Sometimes, February misses out on a Full Moon, but it's the only month that can.
How often does February not have a Full Moon? This happens in years with double Blue Moons, so only four or five times in a hundred years.
How accurately can you calculate the time of a Full Moon? To within a second or so, but the Earth is spinning erratically, which makes long-term predictions much more uncertain.
When did the 2001 Blue Moon occur? October, November or December, depending on your timezone.
How often is there a Blue Moon (or a Full Moon) on Halloween? A Full Moon on Halloween must also be a Blue Moon, and it happens five or six times during the 21st century.
What about double New Moons? They occur every two or three years, just like double Full Moons, but fewer people notice them.
Who wrote these pages, and why should we believe what we read here? His name is David Harper. He lives near Cambridge, England with his wife and two cats. He has a Ph.D. in mathematics and astronomy. He used to be a professional astronomer, but now he just does it for fun.
When will the next Blue Moon occur? We have an interactive Blue Moon Calculator which will tell you whether your favourite year has a Blue Moon.

If your question isn't in the list, then please contact us. We would love to hear from you!


Obliquity is proud to be a partner with Floris Books in the creation of The Stargazers' Almanac 2012, your monthly guide to the stars and planets.
The Stargazers' Almanac 2012


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Last modified on 31 March 2012
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