Why are there 13 Full Moons this year?

Lunar months are actually exactly 29.53 days long, while most months are longer than 29 days.

Consequently, the date of the Full Moon drifts to be sooner in successive months.

There inevitably comes a time when there is both a Full Moon on the first and last day of the month.

This is something that in 2020 occurs in October.

The second Full Moon of that month, on October 31, will be the Blue Moon.

So this year will actually have 13 Full Moons between January and December.

The Farmers’ Almanac said: “In our lexicon, we describe an unusual event as happening ‘Once in a Blue Moon’.

“This expression was first noted back in 1821 and refers to occurrences that are uncommon, though not truly rare”.