When do you call a full moon “blue moon”?

Everyone knows that about once a month is full moon … but, strangely enough, a full moon is always on a different day each month and always at a different time. The reason is that the moon month is shorter on the average, than the calendar month. Without being to mathematical, one can say that a moon cycle lasts approx. 29.5 days. This is how the time of full moon spreads irregularly throughout the calender months and it happens (about every 2.5 years) that two full moon dates fall onto the same month, one at the beginning and one at the end of the month. This second full moon is called a “blue moon” today.

However, it was not always like that. In the 19th century, the Farmer’s Almanac in Maine, US, described the third full moon within one season of four full moons as a “blue moon”. It was only in 1946, that the nowadays widespread definition of the “blue moon”, being the second full moon within one calendar month, was determined, due to a false interpretation by James Hugh Pruett (1886-1955), in an astronomic article of the magazine Sky & Telescope.

February presents a special case, as it is the only month with 28 or 29 days that is shorter than the moon month and simultaneously surrounded by two months with 31 days. When no full moon occurs in February, there will be a full moon at the beginning and end of January, as well as at the beginning and end of March. Thus, a blue moon in January, no full moon in February and another blue moon in March. The last time this happened was in 2018, the next time will be in 2037 (recurs approx. every 19 years):

• Full moon on 2 January 2018 and 31 January 2018
• No full moon in February 2018
• Full moon on 2 March 2018 and 31 March 2018

• Full moon on 2 January 2037 and 31 January 2037
• No full moon in February 2037
• Full moon on 2 March 2037 and 31 March 2037

But why is it called “blue moon” in the first place? Particularly, when it is not at all blue. Various explanatory models and hypothesis exist of its origin. On one hand, history speaks quite frequently about a full moon appearing in a special bluish light shortly after a volcano eruption. On the other hand, the phrase “once in a blue moon” is used to describe a rare event. Yet, this could derive from an Old English poem of 1528, which states:

“Yf they say the mone is blewe
We must believe that it is true.”

We would point out that the blue moon does not look differently to any other full moon (and particularly not blue), but will only be special in our thoughts.

Here are the blue moon dates for 2000–2050 (time specified in Central European Time CET or Central European Summer Time CEST):

Thursday, 1st Nov 2001, 06:41:00 am
Friday, 30th Nov 2001, 09:49:00 pm (= blue moon)

Friday, 2 July 2004, 01:08:54 pm
Saturday, 31 July 2004, 08:05:06 pm (= blue moon)

Friday, 1 June 2007, 03:03:36 h
Saturday, 30 June 2007, 03:48:42 pm (= blue moon)

Wednesday, 2 Dec 2009, 08:30:18 am
Thursday, 31 Dec 2009, 08:12:36 pm (= blue moon)

Thursday, 2 Aug 2012, 05:27:30 am
Friday, 31 Aug 2012, 03:58:06 pm (= blue moon)

Thursday, 2 July 2015, 04:19:36 am
Friday, 31 July 2015, 12:42:54 pm (= blue moon)

Tuesday, 2 Jan 2018, 03:24:06 am
Wednesday, 31 Jan 2018, 02:26:48 pm (= blue moon)

Friday, 2 Mar 2018, 01:51:24 am
Saturday, 31 Mar 2018, 02:36:54 pm (= blue moon)

Thursday, 1 Oct 2020, 11:05:18 pm
Saturday, 31 Oct 2020, 03:49:12 pm (= blue moon)

Tuesday, 1 Aug 2023, 08:31:42 pm
Thursday, 31 Aug 2023, 03:35:42 am (= blue moon)

Friday, 1 May 2026, 07:23:12 pm
Sunday, 31 May 2026, 10:45:12 am (= blue moon)

Monday, 1 September 2031, 11:20:30 am
Tuesday, 30 September 2031, 8:57:54 pm (= blue moon)

Friday, 2 January 2037, 3:35:18 am
Saturday, 31 January 2037, 3:04:12 pm (= blue moon)

Monday, 2 March 2037, 1:28:12 am
Tuesday, 31 March 2037, 11:53:36 pm (= blue moon)

Friday, 1 August 2042, 7:33:24 pm
Sunday, 31 August 2042, 4:02:24 am (= blue moon)

Thursday, 1 September 2050, 11:30:54 am
Friday, 30 September 2050, 7:31:48 pm (= blue moon)

If you do not want to miss this date, simply register to receive our free full moon memo and you will be reminded via e-mail.


  1. Ty | 28 February 2018

    Ty for such intellectual atticles! I’ve been
    I’ve learned a lot, thanks to your articles.

  2. Lynne Werner | 31 January 2018

    It has been very interesting information that u provide for us . Thanks again for your hard work and knowledge.

  3. Sue Merrell | 30 July 2015

    I believe the moon affects behavior. I’ve seen it a hundred times in my newspaper career. That’s the subject of my latest mystery Full Moon Friday. In honor of the blue moon, you can download a free Kindle copy of Full Moon Friday today and tomorrow. No kindle? You can read it on a smart phone or tablet with a free kindle app. Enjoy the blue moon!

  4. Laurie | 29 July 2015

    Is there any celebration or activity anybody can suggest to make an outdoor party themed around the “blue moon”

    • Stella | 3 August 2015

      This is a good idea. I am in the UK. Anyone in the UK interested to form our community of those like minded for planetary celebrations, teaching the phases of the moon and its effect on our entire body, and ofcourse gardening.

  5. Evelyn | 29 July 2015

    Is there a name for a month that has no full moon, such as February 2018 will be?

    • Jörg & Pat | 29 July 2015

      We have not found any name for a calendar month without a full moon.

    • Marie Wise-Miu | 29 July 2015

      That’s what I was wondering! It seems to me that the no moon month is a rarer event than the blue moon. It should have it’s own name. Maybe we should start one. What do you think? What name would be good for that? 🙂

    • Bob | 29 July 2015

      black moon

  6. Elly Hinz 29. july | 29 July 2015

    Thank you so much for all the wonderfull information,
    I really love it.

  7. Margaret | 27 October 2012

    I saw the blue moon in Mallorca. Will send pic.
    Margaret, Solis Clairvoyant Glasgow Scotland.

  8. Lil | 1 September 2012

    I couldn’t see the blue moon last night 🙁 I wish I could!
    I think am so affected by the moon, I have a terrible headache today and yesterday I had different emotions!

  9. Robert | 31 August 2012

    3 days before, the day of and three days after. I am truly affected by the FULL MOON, and I remind people at work almost every time it comes around…I’m not surprised by their responses, they realize at that moment that a change in their mood is taking place…I reminded friend today, and her response was “So that’swhy I been a Bitch today!” And believe me!! she can act up from time to time, but I love her to death. I am aware of everything that goes on in me, my Soul! And right now I feel the moon, a bit hyper, but I feel good and know why…2u2

  10. Karthick GJ | 31 August 2012

    Very intresting message. Makes me to show more love on “My Dear Moon”.

  11. Judie Ann | 30 August 2012

    August 31st is my birthday. Children of the moon arise!

  12. Jillian | 30 August 2012

    I too am affected by the moon. Please do not try to inhabit this glorious
    territory, haven’t we done enough damage?

  13. nikki weber | 29 August 2012

    I love the moon, I am truly a child of the moon and I am always greatly affected by the full moon, I feel the blue moon is an exceptional time for me, so I can’t wait to enjoy this blue moon. I will be in Newport, Oregon dancing by the light of the moon.

    • Celeste | 9 September 2012

      Indeed, nature is wonderful. And yes, new moons and the dark skies they create offer some amazing stargazing opportunities. Here’s hoping for clear skies over the holidays so that folks can see the Milky Way. As we say at PPFF, You know you have been away from nature too long if you think the Milky Way is just a candy bar. Do you have any stargazing tips for people to help them enjoy their time looking at the night skies?

  14. Angela Leger | 29 August 2012

    Thank you so much for all the information given concerning the moon. I really appreciate it. Great job on your parts.

    • Dilek | 9 September 2012

      Full moons? Bah! New moons are where it’s at—for stargazers, at least. The next few new moons (which means no pesky moihgolnt ruining our view of the stars) are July 1, July 30, Aug. 28, and Sept. 27. Five days either side of a new moon means great stargazing at Cherry Springs and many other PA state parks. The Fourth of July weekend will be a great time to see the Milky Way if the skies are clear—which is another thing to consider. Isn’t nature wonderful?

  15. Mira | 29 August 2012

    NEW YORK = Friday * 31st August 2012 * 09:58:06 am (EDT)
    I have to look at full moon Thursday/Friday night or Friday/Saturday?
    Thank you

    • Jörg & Pat | 29 August 2012

      You can take both nights, but from Thursday to Friday you are closer to the full moon.

  16. Lisa | 5 August 2011

    So the next ‘blue’ moon is 31 August 2012 at 13:59 (I live in England). Does anyone know if I will I be able to tell when it gets dark that its ‘full’? And if so, should I be looking in the dark the night before, or after 13:59 on 31 August? (I realise it will not truly be a full moon but will it look full to an untrained eye?)

    • Jörg & Pat | 9 August 2011

      For that day, moonrise in London will be around 7:16 pm (sunset around 7:48 pm). If the weather is good you will be able to see a ‘full’ moon the night before and after this ‘high noon full moon’ (exact astronomical point in time 12:59 pm for London). You are right, it is not possible to see a difference between a 99% and a 100% circle.

      But as we mentioned in our article: there will be nothing special about this full moon. It will not appear blue at all. You can look at any other full moon!

  17. Joanne | 13 July 2011

    When is the Full moon in Ontario Canada?

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