Explainer
Each month’s full Moon has a well-known name, but this year they get really mixed up
Today everyone’s familiar with the popular, informal names given to full Moons – there’s a different one for each month – June’s full Moon is called the Strawberry Moon. But there’s a problem with these names, because our calendar of 12 months isn’t in sync with the lunar cycle – and in 2020 it really comes home to roost.
For the most part our calendar and the lunar cycle are in sync and we get a full Moon every month, but sometimes there are two full Moons in one month and possibly none in another. And this mismatch gets more complicated because one full Moon does have a fixed definition – the Harvest Moon. These problems are illustrated by this year’s full Moons (see list, right) and the way the popular names fall.
Most of the names fit just fine, but there’s one gap and it’s not easy to fill – that’s the Harvest Moon. The Harvest Moon is defined as the closest full Moon to the Northern Hemisphere’s autumn equinox, and this year, the full Moon on 1 October gets the title because it occurs just nine days after the equinox. But search online for ‘Harvest Moon 2020’ and you’ll see many references to it being the 2 September full Moon, which is 20 days earlier than the autumn equinox; this is simply because of where it falls in the informal naming sequence.
So how is this gap in popular full Moon names in 2020 to be filled? One compromise would be to call it the Corn or Barley Moon, names sometimes used for the September full Moon, but ignored when the Harvest Moon occurs in September.
Once in a Blue Moon?
The Harvest Moon mix-up also raises questions about another lunar term around which there is an element of confusion, the Blue Moon.
By some definitions, the full Moon on 31 October is a Blue Moon as it’s the second full Moon in the same month. This definition has become a popular one, but it’s based on a misinterpretation which originated in a 1946 edition of Sky & Telescope Magazine and became widespread in the 1980s after it was used in a popular radio show and the board game Trivial Pursuit. The original definition of a Blue Moon is the third full Moon in a season which contains four full Moons. Here, the seasons are defined as the time between winter solstice, spring equinox, summer solstice and vernal equinox. The list below shows how things break down for 2020.
The full Moon which occurs on 30 December 2020 technically belongs to the new winter season for 2021 because it falls after the Northern Hemisphere’s winter solstice on 21 December 2020. So rather eloquently, if you follow the original definition, there is no Blue Moon in 2020 because each season only has three full Moons.
So with no formal definitions for full Moon names, except the Harvest Moon, and with the mistaken definition of Blue Moon, the popular full Moon naming system is showing its cracks this year. But there is a solution that fixes the names of the full Moons and the definition of a Blue Moon and marries them together. This is an antique naming system, that was originally used in old almanacs of the British Isles. These poetic names (see list, below) are attributed by season.
The waxing and waning appearance of the Moon is a constant in our skies and the appearance of the full Moon can be a mixed blessing. Its bright light makes it much harder to see the stars beyond, but it does have a beauty all of its own – something easy to appreciate if you take the time to look. Using informal or formal names helps us appreciate the lunar cycle, and it’s interesting to see how the naming systems had the mismatch between our calendar and the lunar cycle covered. Maybe this is a better way to go in the future.