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

<p>From Wikipedia:</p>

<p>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.</p>

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

CC BY-SA 3.0
2016-07-19T11:34:31.037Z
Does that allow for no blue moons in February?  perhaps somebody could bum this to Maths Stack Exchange?
Robert Frost
CC BY-SA 3.0
2016-07-19T11:50:45.063Z
As its only an average it should take into account February.
James Screech
CC BY-SA 3.0
2016-07-19T12:22:34.833Z
If there's no full moon in Feb, then a blue moon can occur before 2.716 years has elapsed
Robert Frost
CC BY-SA 3.0
2016-07-19T13:26:30.383Z
As that is an average value then yes it could be less or more depending on the exact circumstances.
James Screech
CC BY-SA 3.0
2016-07-19T13:31:21.603Z
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](http://eclipse.gsfc.nasa.gov/SEhelp/image/Fig4-3a.png).
David Hammen
CC BY-SA 3.0
2016-07-19T14:17:57.403Z
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 (http://www.moongiant.com/Blue_Moon_Calendar.php) this will happen again in 2018.
Donald.McLean
CC BY-SA 3.0
2016-07-19T20:14:14.487Z
@Donald.McLean yes you've got the point. This will happen quite frequently and therefore the stated average is quite inaccurate.
Robert Frost
CC BY-SA 4.0
2022-09-11T23:35:48.850Z
The Wikipedia page is describing how to calculate the frequency of a "season blue moon" - that is, a fourth full moon within a three month period.  This calculation is not correct for a "month blue moon" - that is, a second full moon within a single calendar month.
Dawood ibn Kareem
CC BY-SA 3.0
2016-07-19T10:52:11.617Z
2016-07-19T11:34:44.407Z
2016-07-19T11:34:44.407Z
James Screech
<p>Be aware of the other kind of event also referred to as "a blue moon". </p>

<p>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.</p>

<p>These "blue moon" events are very rare, and can not be predicted. </p>

CC BY-SA 3.0
2016-07-21T03:29:57.203Z
2016-07-21T03:29:57.203Z
Cam_Aust
<p>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?</p>

CC BY-SA 3.0
2016-07-19T17:24:40.040Z
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
CC BY-SA 3.0
2016-07-19T20:28:14.900Z
@BillOer thanks, good point. I defined the meaning I'm interested in just to be sure.
Robert Frost
<the-moon><moon-phases>
How often is a blue moon?
Robert Frost
# How often is a blue moon?

(Asked by: username_0 on 2016-07-19)

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?

**Tags:** the-moon, moon-phases
> **Comments:**
> **username_1**: 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.  
> **username_0**: @BillOer thanks, good point. I defined the meaning I'm interested in just to be sure.  

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## 2 answers

### Answer by username_2 on 2016-07-19. Score: 3

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.

> **Comments:**
> **username_0**: Does that allow for no blue moons in February?  perhaps somebody could bum this to Maths Stack Exchange?  
> **username_2**: As its only an average it should take into account February.  
> **username_0**: If there's no full moon in Feb, then a blue moon can occur before 2.716 years has elapsed  
> **username_2**: As that is an average value then yes it could be less or more depending on the exact circumstances.  
> **username_3**: 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](http://eclipse.gsfc.nasa.gov/SEhelp/image/Fig4-3a.png).  
> **username_4**: 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 (http://www.moongiant.com/Blue_Moon_Calendar.php) this will happen again in 2018.  
> **username_0**: @Donald.McLean yes you've got the point. This will happen quite frequently and therefore the stated average is quite inaccurate.  
> **username_5**: The Wikipedia page is describing how to calculate the frequency of a "season blue moon" - that is, a fourth full moon within a three month period.  This calculation is not correct for a "month blue moon" - that is, a second full moon within a single calendar month.  

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### Answer by username_6 on 2016-07-21. Score: 0

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.


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