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
- Four full moons in a season
The first definition refers to a fourth full moon in a season. Everybody knows that 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 reason why the year has 12 calendar months.
The Ancient Romans had 12 moon cycle and then a period of rest (roughly what we call now January and February) until the new year started again in March (which coincided with the start of the military marching season, ruled by Mars, the God of war). But 12 moon cycles don’t add up to 365 days. To keep the year in sync with the seasons occasionally another moon cycle was added, which then brought the total number of full moons in a year to 13. This meant that one season had 4 full moons instead of 3 and the 4th full moon 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).
Although the sun calendar replaced the former lunar model for official duties and taxes, many pagan rituals were still celebrated in accordance to the moon cycle. When the Emperor Constantin adopted Christianity as the ‘official Roman religion’ anything pagan got a bad press. The Catholic church rallied against the ancient pagan practises and the number 13 became the ‘number of witches’. It was hailed the unlucky number, especially 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 how often this happens: The last ‘Blue Moon’ according to this definition occurred just recently on the 21st May 2016 and the next ‘blue moon’ will happen on 18th May 2019 and after that 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 which 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. 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 is a bit more random than the rhythym of the 13 moon cycle. The last time this happened was in March 2016 and there were no Blue Moons in 2017.
But now we start the year 2018 with a Blue Moon in January (31st), no full moon in February and then again another Blue Moon in March 2018 (31st March). After that the next Blue Moon will happen in October 2020.