What’s in a blue Moon?

There are two full Moons in October 2020, one on 1 October and one on the 31 October. Popular culture calls the second full Moon in a month a ‘blue Moon’, but it’s a term that has been misused over the years.

The original Maine Farmers’ Almanac definition of a blue Moon says the term should refer to the third full Moon that occurs in a quarterly season of four: a season defined as winter solstice to spring equinox, spring equinox to summer solstice, summer solstice to autumn equinox and finally autumn equinox to winter solstice.

If the original definition is adhered to, there is no blue Moon in 2020 as all four seasons only have three full Moons.

The full Moon on 1 October does have a formal title. The nearest full Moon to the Northern Hemisphere’s autumn equinox is the Harvest Moon.

This year the autumn equinox occurred at 14:30 BST (13:30 UT) on 22 September. The full Moon prior to this occurred at 06:22 BST (05:22 UT) on 2 September, 20 days, 8 hours and 8 minutes from the equinox.

The full Moon on 1 October takes place 9 days, 6 hours and 36 minutes from the equinox, which defines the 1 October full Moon as the Harvest Moon for 2020.

If you’re wondering how a full Moon is defined, it’s to do with the position of the Moon in the sky relative to the Sun.

When the Moon is full it’s technically at opposition, meaning its ecliptic longitude must be 180˚ from the ecliptic longitude of the Sun.

Find out more about what to see on the Moon with our guides How to observe the Moon and Why does the Moon’s appearance change?.

Pete Lawrence is an experienced astronomer and a co-host of The Sky at Night. This guide originally appeared in the October 2020 issue of BBC Sky at Night Magazine.