Once in a Blue Moon

When you hear someone say, "Once in a blue moon …" you probably think that they are talking about something rare. In fact, this month’s image shows the blue moon of October 2020 sitting above the summit of Càrn na Nathrach in Ardgour, with the peaks of Druim Garbh, Sgùrr Dhomhnuill and Sgùrr na h-Ighinn to its left. So, what exactly is a blue moon and is it actually that rare?

Hunter’s Blue Moon: Loch Shiel viewed from Cnoc Breac, Mingarry, Moidart
The second full moon of October 2020, a Hunter’s Blue Moon, rising over the summit of Càrn na Nathrach, making its way south towards the peaks of Druim Garbh, Sgùrr Dhomhnuill and Sgùrr na h-Ighinn

​Most people will have watched a white coloured moon traversing the night sky and some people may have even seen a yellow or red moon at moonrise or moonset. On the other hand, very few people will have seen a blue coloured moon because it is a very rare occurrence indeed.
 
This rare event, when the Moon can naturally appear blue, is caused by the presence of dust particles in the atmosphere which, if they are the right size, can scatter the red part of the light spectrum and leave the rest untouched to cause the moon to take on a blue tint.
 
Some of the events that have led to this include the eruption of Krakatoa in 1883 and the eruptions of Mount St. Helens in 1980 and the El Chichón volcano in Mexico in 1983. In fact, the eruption of Krakatoa reportedly caused blue moons for nearly 2 years.
 
However, the bluish tint caused by these rare events doesn’t have anything to do with the phrase “once in a blue moon”. Instead, the words ‘blue moon’ are used to describe the second full moon in a calendar month. It is an event that is not that rare because there are 29.5 days between full moons, meaning that each year will have roughly 12.3 full moons. This gives us a 13th full moon once every 2.7 years, along with a month within which a second full moon, or blue moon occurs.
 
In fact, the last blue moon to occur was the one that is shown in the image above, which I photographed in October 2020. The next one is in August 2023 and the one after that will be in May 2026.
 
Considerably rarer than blue moons are double blue moons. This is when there are two blue moons in the same calendar year, and it is something that occurs about 3-5 times every hundred years of so. The last one was in 2018, the next is expected in 2037 and the one after that in 2067.
 
Rarer still is what is termed a super blood blue moon. This is when a blue moon occurs when the Moon comes within 90% of its closest distance to Earth to make it a supermoon, while at the same time fully slipping into the Earth’s shadow to create a total lunar eclipse. This last happened in January 2018, just over 150 years since all three phenomena last lined up in March 1866.
 
If you look at the frequency at which these three phenomena are expected to occur;
 

• blue moons make up about 3% of all full moons;
• supermoons are approximately 25% of all full moons; and
• total lunar eclipses occur during 5.6% of full moons;

 
meaning that a super blood blue moon will occur 0.042% of the time, or once every 2,381 full moons. On average, that corresponds to once every 194 years!
 
So, the next time you think about using the phrase “once in a blue moon” for something that almost never happens, you might want to consider using “once in a super blood blue moon” as an alternative!!