How often is a blue moon?
| “Once in a blue moon”? A blue moon occurs about once every two or three years on average. But this is not exactly regular. The year 2018 for example had 2 blue moons in the same year! Find out more about blue moons and some of the details and misconceptions surrounding them below. |
Contents
What is a blue moon?
You have probably heard of the common expression, “once in a blue moon”, usually referring to an event or occurrence that is rare.
Blue moons are not as rare as you might think though.
But what exactly is a blue moon?
The first thing to note here is that the term blue moon, as used today, has nothing to do with the colour of the moon.
Sure, the moon can sometimes appear to us as blue. As it can also look grey, yellow or red.
But this has nothing to do with the colour of the moon changing – it is always the same. It simply has to do with dust or particles in the air of our atmosphere making is seem that way.
So the “blue moon” looks in no way no different to any other ordinary full moon.
There is A LOT of confusion surrounding the exact meaning and definition of the term.
| By definition(s), a blue moon actually refers to: An extra full moon in a part or subdivision of a calendar year: 1. a 2nd full moon in one month in the standard calendar (the NON TRADITIONAL definition) OR 2. the 3rd full moon of four full moons in an astronomical season (the TRADITIONAL definition). |
Yeah, I know, not so simple.
And disputed, debated on for many years. So it is not just you and me that find it confusing.
But why have in the first place?
Why do blue moons exist?
The reason for them is very simple.
Since the dawn of human civilisation and all over the world, people have taken note of and given names to full moons. People also noticed they occur about once per month.
Given 12 months in a year, 12 full moons were accounted for.
This is where it gets interesting.
The rotation of the moon phases takes about 29 and a half days (29.53 to be exact) in total. That would then equal to about 354 days for 12 cycles.
But this is shorter than the 365 or 366 days in our calendar year! Therefore, we need an extra full moon. You guessed it, it was named the “blue moon“.
This is a bit similar to the reason for leap years occurring we discussed previously for example.
How often does a blue moon occur?
Using the maths above, this would mean that we need a blue moon once every 2 or 3 years!
And that is exactly the case. On average.
When it occurs precisely date wise though, will depend on the definition of blue moon you use. And also where you are in the world – due to the different time zones globally and the fact that the moon is only full in a single moment.
Occasionally, you will find some years that actually have 2 full moons in one year! This happened in year 2018. The next time will be in year 2037.
Blue moon dates
By the non traditional or calendar definition (2nd full moon in one month), the dates are:
31st October 2020
31st August 2023
31 May 2026
31 December 2028
By the traditional or seasonal definition (3rd full moon of four full moons), recent ones were on:
May 21 2016
May 18 2019
August 22 2021
When is the next blue moon?
Following on from the above, the next calendar blue moon is on the 31st of August of 2023 and seasonal on the 22nd of August of 2021.
Did you find this article useful?
Or do you have any suggestions or important details to add?
Please let us know in the comments below.
References
Once in a Blue Moon, Smith, Bridey – The Age Australia, 2009, accessed online in November 2020, link here
Blue Moon, Wikipedia page, accessed October 2020, link here
Eclipses and the Moon’s Orbit, Fred Espanek, NASA website, accessed online December 2020, link available here
This article on how often is a blue moon is created for informational purposes only. We hope you find it useful along with the other astronomy and meanings section articles but note we do not accept any responsibility for any liability arising from the use of the information, even if deemed inaccurate, as outlined in the terms of use of How Often Should website.
