What is a blue moon?

What is a Blue Moon

You’ll no doubt have heard the phrase “once in a blue moon”, but have you ever wondered where it came from, and exactly what is a blue moon?

If you look up in the sky tonight, 31st July 2015, you’ll see the full moon. If you look up at the moon often, you may have noticed there has already been a full moon during July. So this being the second in a month, is called a Blue Moon.

Metonic Cycle

A Greek astronomer called Meton, observed that a 19 year cycle (228 calendar months) was almost exactly the same as 235 lunar phases.  So after those periods, the moon phases will start again at almost exactly the same time and dates of the year.

Therefore the next July to have a blue moon will be in 19 years time – 2034 – then 19 years after that, it will happen again in July.  This cycle was named the Metonic Cycle after Meton.

As there are 235 full moons over the 19 year (228 month) period, that means we will have 235 – 228 = 7 more full moons than calendar months.  However, as February has only 28 days, in some years that month will not have a full moon at all.

Therefore the extra full moon that happens just before, or just after February will occur in another calendar month with another full moon, thereby creating an additional “blue moon”.  This will next happen in 2018 – where both January and March of that year will have a second full moon on the 31st of each month.

So in this next metonic cycle, we will have 8, instead of the expected 7 blue moons:

  • 31st January 2018
  • 31st March 2018
  • 31st October 2020
  • 31st August 2023
  • 31st May 2026
  • 31st December 2028
  • 30th September 2031
  • 31st July 2034

Seasonal Blue Moon

This 19 year cycle also produces 7 seasonal blue moons – which instead of being a second full moon in a calendar month, the seasonal blue moon is the 3rd of 4 full moons in one season.

The last seasonal blue moon was on 21st August 2013. During this metonic cycle, the next 7 will be:

  • 21st May 2016
  • 18th May 2019
  • 22nd August 2021
  • 19th August 2024
  • 20th May 2027
  • 24th August 2029
  • 21st August 2032

Short Video of our Moon

by Matti Svensson


So the next time your hear the phrase “once in a blue moon”, or someone asks you “what is a blue moon”, you’ll know that it means something which is rare – and in astronomy terms, something that happens only 7 or 8 times in a 19 year period.


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