When do you call a full moon »blue moon«?

Everyone knows that about once a month is full moon … but, strangely enough, 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 »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 »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.

Februrary 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 next time this will happen, will be in 2018 (recurs approx. every 19 years):

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

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 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 2000–2030 (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, 2nd July 2004, 01:08:54 pm
Saturday, 31st July 2004, 08:05:06 pm (= blue moon)

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

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

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

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

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

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

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

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

Friday, 1st May 2026, 07:23:12 pm
Sunday, 31st May 2026, 10:45:12 am (= 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.

Column: Worth reading | 25 comments | Leave a comment
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Comments

  • 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!

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

    • 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.

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

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

    • 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? :)

    • black moon

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

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

  • 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!

  • 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

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

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

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

  • 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.

    • 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?

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

    • 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?

  • 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
    Mira

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

  • 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?)

    • 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!

  • actually.. I believe the term blue moon refers (originally) to the extra moon in a season, not 2nd moon in a month. see http://www.skyandtelescope.com/observing/objects/moon/3304131.html?page=2&c=y

  • When is the Full moon in Ontario Canada?

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