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.
Is there a Blue Moon this year? Next year? The year I was born? The Blue Moon Calculator knows the answer!
Click the button at the right to see a more detailed answer to each question.
Isn't your definition of Blue Moon wrong?
More Well, 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?
More Once every two or three years, just like the new definition, but not in the same years.
Where did the expression Blue Moon come from?
More It's been part of the English language for four hundred years. During that time, it's had half a dozen different meanings.
Does a Blue Moon have any effect on human behaviour?
More It has no more effect on human behaviour than any other Full Moon.
Does the Moon actually change colour during a Blue Moon?
More No, but there have been rare occasions when the Moon really did turn the colour blue.
Why is there a bright ring around the Moon?
More 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?
More 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?
More Four or five times in a hundred years.
Can a month have no Full Moon?
More 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?
More This happens in years with double Blue Moons, so only four or five times in a hundred years.
How often is there a Blue Moon (or a Full Moon) on Halloween?
More A Full Moon on Halloween must also be a Blue Moon, and it happens five or six times during the 21st century.
How accurately can you calculate the time of a Full Moon?
More To within a second or so, but the Earth is spinning erratically, which makes long-term predictions much more uncertain.
When did the 2015 Blue Moon occur?
More July, in all time zones.
When did the 2012 Blue Moon occur?
More August or September, depending on your timezone.
When did the 2009/2010 Blue Moon occur?
More December 2009, January 2010 or March 2010, depending on your timezone.
When did the 2007 Blue Moon occur?
More May, June or July, depending on your timezone.
When did the 2004 Blue Moon occur?
More July or August, depending on your timezone.
When did the 2001 Blue Moon occur?
More October, November or December, depending on your timezone.
What about double New Moons?
More 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?
More His name is David Harper. He has a PhD in mathematics and astronomy, and lives near Cambridge, England. He used to be a professional astronomer, but now he just does it for fun.
When will the next Blue Moon occur?
More We have an interactive Blue Moon Calculator which will tell you whether your favourite year has a Blue Moon.
How do I ask a question that isn't on this list?
More We would love to hear from you!