Not a ‘blue Moon’

BEST TIME TO SEE: 1 & 31 October

There are two full Moons this month, one on 1 October and one on the 31st. Popular culture calls the second full Moon in a month a ‘blue Moon’, a term which has been misused over the years. The original Maine Farmers’ Almanac definition of a blue Moon should refer to the third full Moon which 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.