Have you ever heard the phrase – once in a blue moon. I should think so. So I delved a little deeper to find out what it really means and where it comes from
Countless poems and songs have been written about the Blue Moon, meaning an event that does not come around too often.
The phrase ‘Blue Moon’ became widespread in the late 19th century. One of the reasons could be that there was an actual event when the moon really turned blue. The date was the 26th of August 1883. On this day, a massive eruption occurred on the Indonesian island of Krakatoa.
The death toll counted over 34,000 people and many more injured. A number of tsunamis followed and the shocks were felt as far as Australia. Two-thirds of the island of Krakatoa was completely destroyed and debris was found as far as Madagascar.
As a result, an ash column stretching over miles entered the atmosphere. It subsequently lowered the world temperature by over 1.2 degrees. But the positive byproducts were spectacular orange sunsets and the moon had a blue-greenish tinge. This phenomenon lasted for a few years and the phrase Blue Moon was born.
Disturbingly, in early 2020 Krakatoa shows signs of activity again and there was even a small eruption in April, but nobody was hurt.
We know a Blue Moon is a rare occurrence – but how often does this actually happen and how is it calculated? There are 2 definitions.
1. Four full moons in a season
The first definition refers to a fourth full moon in a season. The year has four seasons: spring, summer, autumn and winter and normally each season has 3 full moons. So that brings the yearly total to 12 full moons and the main reason why the year has 12 calendar months.
The Ancient Roman year was comprised of 12 moon cycles and then a period of rest, roughly what we now call January and February which did not exist at that time.
The new year started again in March. This coincided with the new military marching season, ruled by Mars, the God of war. To keep the year in sync with the seasons, very occasionally an additional moon cycle was added to the year. This meant there were now 13 months (moon cycles) instead of the usual 12. As a result, one season had 4 full moons instead of 3. The last full moon in that season was called a Blue Moon.
When Julius Caesar adopted the solar calendar model, he created 12 calendar months per year. (calendar comes from the Latin word for register) and abolished the 13th lunar month altogether. Instead, he opted for leap days every 4 years.
Although the new Roman sun calendar replaced the former lunar model for official duties and taxes, many pagan rituals were still celebrated in accordance with the moon cycle. About 400 years later, Theodosius I made Christianity the official Roman religion. The table very quickly turned. Less than 100 years earlier Christians had been persecuted for their belief and public executions often followed.
Under Theodosius I, now Christian heretics and people who did not adopt this new state religion found themselves in the same position. Any pre-Christian practices such as fertility rituals, worship of nature and following the lunar instead of the new sun calendars were outlawed.
The number 13 became synonymous with outdated knowledge and seen as evil. It was hailed double unlucky if the combination fell on a Friday (the day of worship for Friga, the pagan fertility goddess). This lore is still alive in fairy stories. Do you remember the 12 good fairy godmothers in Sleeping Beauty and the 13th came to dinner and spoilt it all?
To give you an idea of how often this happens – The last ‘Blue Moons’ according to this definition occurred on the 21st May 2016 and 18th May 2019. The next Blue Moon is due on the 22nd August 2021. So on average, a Blue Moon happens every 3 years, hence the saying ‘ once in a blue moon’, meaning an event that is very rare.
2. Two full moons in a calendar month
Another way of describing a ‘blue moon’ came later. Normally a calendar month has one New Moon and one Full Moon, but occasionally 2 of each can occur in the same month. From the 19th century onwards it became popular to call the second Full Moon in a given calendar month ‘a blue moon’. Although still rare, this event occurs more randomly than the first definition. The last time this kind of Blue Moon happened was in March 2016 and there were no Blue Moons in 2017. 2018 had a Blue Moon in January (31st), no full moon in February, and then again another Blue Moon in March (31st March). After that, the next Blue Moon happens on the 31st October 2020 which incidentally is also Halloween.