The last Blue Moon fell on the night of August 20-21, 2013. Read more about this Blue Moon here. It was a Blue Moon by the seasonal definition, that is, the third of four full moons to take place in a season, in this case between the June 2013 solstice and September 2013 equinox. 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.
Are only two full moons in a three-month season possible? Yes, it’s possible, but it’s extremely, extremely rare! It last happened during the Northern Hemisphere winter (Southern Hemisphere summer) of 1961/1962, 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)
If there are four full moons in a season, one of them is a Blue Moon. Usually, there are three full moons in one season – a season being defined as the period of time between a solstice and an equinox, or vice versa. Sometimes, though, a season can have as many as four full moons.
This year, for instance, four full moons fall between the June 2013 solstice and the September 2013 equinox. 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 sometimes called a Blue Moon.
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)!
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:
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 August 2013 Blue Moon article.
You may have noticed that there is only one season where a two-full-moon season is possible: Northern Hemisphere winter (Southern Hemisphere summer). That’s because a Northern Hemisphere winter/Southern Hemisphere summer gives us the shortest season of the year, lasting about 89 days. Spring lasts for nearly 93 days, while summer lasts for nearly 94 days and autumn 90 days.
For the 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!
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!