The origin of the curious phrase “once in a blue moon” is not known for certain (see references below for details).
Even the meaning isn’t agreed upon.
Some say it is like the idiom “when pigs fly”, meaning, “never”, as in “I will stop drinking coffee when the Moon is blue”. Others say it means “rarely” or “infrequently”, as in, “Oh I drink tea once in a blue moon”.
At the time of writing, most people probably take it to mean something which is “rare” or “infrequent”.
The phrase itself, however, isn’t all that rare. Google Books Ngram Viewer shows that it’s been around since the 19th century, and has been trending since about the 1980s.
For our purposes we define a blue moon simply as “the second Full Moon in a calendar month”.
The interval between two Full Moons is about 29.5306 days (a synodic month). All months – except February – have at least 30 days, so it is possible for there to be two Full Moons per month. But because February has at most 29 days, there can never be a second Full Moon in February.
The following table lists all the blue moons between 2021 and 2200.
2020 Oct 31
2023 Aug 31
2026 May 31
2028 Dec 31
2031 Sep 30
2034 Jul 31
2037 Jan 31
2039 Oct 31
2042 Aug 31
2045 May 30
2048 Jan 31
2050 Sep 30
2053 Jul 30
2056 Mar 31
2058 Oct 31
2061 Aug 31
2064 May 30
2066 Dec 31
2067 Mar 30
2069 Sep 30
2072 Jun 30
2075 Apr 30
2077 Oct 31
2080 Jul 31
2083 May 31
2085 Dec 31
2088 Sep 30
2091 Jul 30
2094 Jan 31
2094 Apr 30
2096 Oct 31
2099 Aug 30
2102 Jun 30
2104 Dec 31
2107 Oct 31
2110 Jul 31
2113 Jan 31
2113 May 30
2115 Dec 31
2118 Aug 31
2121 Jun 30
2124 Jan 31
2126 Oct 31
2129 Jul 31
2132 Mar 31
2134 Dec 31
2137 Aug 31
2140 May 31
2143 Jan 31
2143 Mar 31
2145 Oct 30
2148 Jul 31
2151 Apr 30
2153 Dec 30
2156 Aug 31
2159 Jun 30
2162 Jan 31
2162 Mar 31
2164 Sep 30
2167 Aug 30
2170 Apr 30
2172 Oct 31
2175 Sep 30
2178 May 31
2181 Jan 30
2181 Mar 31
2183 Oct 31
2186 Jul 31
2189 Apr 30
2191 Dec 30
2194 Aug 31
2197 May 31
2200 Jan 31
2200 Mar 31
In this interval (of 180 years) the blue moon happens 73 times. Six years have two blue moons. Most years (113, 63%) do not have a blue moon. So on average, a blue moon happens roughly every three years.
References
- The origin of the phrase “once in a blue moon” (Today I Found Out, 2013 Jun 10)
- Google Books Ngram “once in a blue moon”
- Folklore of the “Blue Moon” (International Planetarium Society, Philip Hiscock, 1999)
- Blue Moon (Wikipedia)
- What’s a Blue Moon? (Sky and Telescope, 2006 Jul 27)
How to cite this webpage
Centre for Astronomical Heritage (2020 September 06) Once in a blue moon. Retrieved from https://cfah.org.za/fullmoon/bluemoon/
Heritage Full Moons in South Africa
| Month | First Full Moon | Second Full Moon |
|---|---|---|
| January | Mantis Moon | Leopard Moon |
| February | Dassie Moon | —— |
| March | Harvest Moon | Ochre Moon |
| April | Diamond Moon | Gold Moon |
| May | Frost Moon | Fire Moon |
| June | Sisters Moon | Honey Moon |
| July | Meerkat Moon | Protea Moon |
| August | Peace Moon | Dusty Moon |
| September | Spring Moon | Blue Crane Moon |
| October | Whale Moon | Elephant Moon |
| November | Milk Moon | Wool Moon |
| December | Springbok Moon | Eland Moon |
