# How often is a blue moon?

A blue moon is two full moons in the same month. How many times will a blue moon occur in the average lifetime of say 70 years?

• There is also another definition of a blue moon; that is the third blue moon in a season that has four rather than the usual three. – BillDOe Jul 19 '16 at 17:24
• @BillOer thanks, good point. I defined the meaning I'm interested in just to be sure. – samerivertwice Jul 19 '16 at 20:28

As months are not of fixed length it is only possible to calculate an average.

From Wikipedia:

The average frequency of a blue moon can be calculated as follows. It is the period of time it would take for an extra synodic orbit of the moon to occur in a year. Given that a year is approximately 365.2425 days and a synodic orbit is 29.5309 days, then there are about 12.368 synodic months in a year. For this to add up to another full month would take 1/0.368 years. Thus it would take about 2.716 years, or 2 years, 8 months and 18 days for another blue moon to occur.

So for a 70 year life span the average number of blue Moons would be 70/2.716 or 25.77.

• Does that allow for no blue moons in February? perhaps somebody could bum this to Maths Stack Exchange? – samerivertwice Jul 19 '16 at 11:34
• As its only an average it should take into account February. – James Screech Jul 19 '16 at 11:50
• If there's no full moon in Feb, then a blue moon can occur before 2.716 years has elapsed – samerivertwice Jul 19 '16 at 12:22
• The length of a synodic month varies, but having two full moons in February would require a synodic month to be almost 13 hours shorter than average. In the 5000 year period centered on 500 CE, the shortest month was less than 7 hours shorter than average. – David Hammen Jul 19 '16 at 13:31
• There was a year during my adulthood where there was a blue moon in January and then another blue moon in March (with no full moon at all in February). According to (moongiant.com/Blue_Moon_Calendar.php) this will happen again in 2018. – Donald.McLean Jul 19 '16 at 14:17

Be aware of the other kind of event also referred to as "a blue moon".

After some large volcanic events, with volcanic ash sent high into the stratosphere and circulating widely around the planet, the moon has appeared blueish in hue rather than the usual reddish, such as seen with total eclipses of the moon.

These "blue moon" events are very rare, and can not be predicted.