The May 21, 2016, full moon carries the name Blue Moon. According to folklore, a Blue Moon can be the second full moon of a month, or it can be the third of four full moons in a season. The May 21 moon is a Blue Moon according to the second definition. So we know there can be four full moons in a single three-month season. But is it possible to have only two full moons in one season? Follow the links below to learn more about seasonal full moons.
Possible to have only 2 full moons in a 3-month season? Yes, it’s possible, but it’s extremely rare! It last happened during the Northern Hemisphere winter (Southern Hemisphere summer) of 1961/2, in between the December 1961 solstice and the March 1962 equinox. A full moon fell shortly before the December solstice in 1961 and after the March equinox in 1962, leaving only enough room for two full moons during the Northern Hemisphere winter of 1961/1962.
Only two full moons in the winter of 1961-1962:
Full moon: 1961 Dec 22 (00:42 UT)
Solstice: 1961 Dec 22 (02:19 UT)
Full moon: 1962 Jan 20
Full moon: 1962 Feb 19
Equinox: 1962 Mar 21 (02:30 UT)
Full moon: 1962 Mar 21 (07:55 UT)
When is the next time a season will have only two full moons? It took some leg work to find out the answer! I directed this query to obliguity.com, and much to my delight, received an immediate response from Dr David Harper. He wrote in an email to EarthSky:
It’s a very rare phenomenon indeed. Between 1962 and 3000, it happens only four more times, in the winters of 2314/5, 2333/4, 2686/7 and 2705/6. In each case, as in 1961/2, there is a full moon less than five hours before the December solstice, and there are four full moons in both the preceding autumn and following spring.
I find it interesting that two lunar cycles seem to be at work when it comes to realigning two full moons with the winter season: the long-period lunar cycle of 372 years and the 19-year Metonic cycle. We elaborate about this 19-year lunar cycle on our article about Blue Moons.
As you might have noticed, there’s only one season where a two-full-moon season is possible: Northern Hemisphere winter (Southern Hemisphere summer). This is the shortest season of the year, lasting about 89 days. Northern spring (southern autumn) lasts for nearly 93 days, northern summer (southern winter) lasts for nearly 94 days, and northern autumn (southern spring) 90 days.
For a winter season to have only two full moons, the December full moon has to occur just before the December solstice.
Also, the full moons from December until March must closely coincide with apogee – the moon’s farthest point from Earth in its monthly orbit. When full moons happen appreciably close to apogee, the time period between successive full moons is longer than average. The shorter season plus longer lunations (lunar months) conspire to give an extremely rare two-full-moon season.
Since the saying Once in a Blue Moon is suppose to indicate something exceedingly rare, or something that almost never happens, I propose that we consider calling the second of a season’s two full moons a Blue Moon! ;-)
Remember, a season, to astronomers, is defined as the period of time between a solstice and an equinox, or vice versa.
In the year 2016, for instance, four full moons fall between the March, 2016, equinox and June, 2016, solstice. Because it’s somewhat rare for four full moons to fit within the framework of a single season, the third of the season’s four full moons is the a Blue Moon.
Why not the fourth one? It’s because each month’s full moons already carry their own names.
Although less than 10% of the seasons harbor four full moons, the occurrence isn’t all that uncommon. A four-full-moon season happens 7 times in 19 years. Or another way of looking at it, a total of 37 four-full-moon seasons take place in the 21st century (2001-2100).
In contrast, a season with only two full moons is truly rare. It last happened during the Northern Hemisphere winter of 1961/1962.
It won’t happen at all in the 21st century (2001-2100), 22nd century (2101-2200) or 23rd century (2201-2300)!
Bottom line: When there are four full moons in a three-month season, the third one is called a Blue Moon. But are there ever only two full moons in a season? Yes, but only extremely rarely. This post discusses the very rare occurrence of two full moons in a single season and proposes a new definition for Blue Moon!
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.