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)

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