What is a ‘Blue Moon’, when can we see it in 2023, and is it really worth the hype?
The full Moon on 30/31 August 2023 will shine big and bright, and is what’s known by some as a ‘blue Moon’.
But how much bigger will August’s blue Moon be compared to an average full Moon, and are blue Moons really worth the hype?
We can get a better understanding of what we really mean by a ‘blue Moon’ by considering what the Moon will be doing for the rest of the month.
August’s blue Moon explained
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The Moon reaches perigee on 2 August at 06:52 BST (05:52 UT). This is the point at which the Moon’s elliptical orbit brings it closest to Earth.
Full Moon is on 1 August at 19:31 BST (18:31 UT), 11 hours and 21 minutes before perigee, making this a perigee full Moon, also known as a proxigean Moon or a supermoon.
A perigee full Moon or supermoon is 14% larger and 25% brighter than an apogee full Moon, although the differences aren’t obvious to the naked eye
In practice it won’t look a lot different to the ones that precede or follow it.
And because there’s a full Moon on 1 August, that means there’s room for a second full Moon within the month of August.
This occurs on 31 August at 02:35 BST (01:35 UT), 9 hours and 44 minutes after perigee on 30 August at 16:51 BST (15:51 UT), making it also a perigee full Moon and the closest of the four occurring this year.
As the second full Moon in a month, it’s known as a ‘Blue Moon’, so this supermoon is dubbed a ‘Super Blue Moon’.
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What is a blue Moon?
A Blue Moon being ‘the second full Moon in a month’ isn’t what the term used to mean, but a misinterpretation that appeared in print in 1946.
As it’s easier to understand than the original ‘third full Moon in a season of four’ meaning, it stuck.
Ironically, full Moons at this time of year tend to rise at a fairly leisurely pace, staying at low altitude.
Consequently, the 31 August Blue Moon is more likely to appear orange than blue!
For more on this, read our guide on why the Moon sometimes appears orange.
This article originally appeared in the August 2023 issue of BBC Sky at Night Magazine.