“Once in a Blue Moon” means this coming Saturday—Halloween. On that date a “Blue Moon” will rise at dusk and set at dawn.
So what is a “Blue Moon,” why do they occur, and when is the next one?
Here’s everything you need to know about this often is understood and very misleading term.
What is a ‘Blue Moon?’
Just to be awkward, there are actually two different types of “Blue Moon.” The most easy-to-understand type happens this Saturday, while the other type is much rarer.
Monthly “Blue Moon”
The second full Moon in a calendar month that contains two full Moons.
Seasonal “Blue Moon”
The third full Moon in an astronomical season (the period between an equinox and a solstice, or vice versa) that contains four full Moons. This is the original scientific definition.
Why are there two full Moons in October 2020?
On Thursday, October 1, 2020 there was a full Moon—the “Harvest Moon.” On Saturday, October 31, 2020—the date of Halloween—there’s also a full Moon, the “Hunter’s Moon.”
It’s because it takes 29.53 days for the Moon to complete one full orbit of the Earth, so it can return to any particular phase in just shy of a month. 12 x 29.53=354.36 days—that’s a lunar year.
How can there be 13 full Moons in the same year?
2020 has an extra full Moon. Since a lunar year is 354.36 days and Earth’s orbit of the Sun takes 365.25 days, every two to three years (precisely 2.7 years, on average) there’s a gap that allows there to be an extra full Moon in a year—so 13 full Moons in total.
When that happens, one of them will be a “Blue Moon.”
According to Timeanddate.com, a “Blue Moon” or one or other kind happens every two or three years, but a monthly “Blue Moon” is slightly more frequent.
In fact, the only month where a monthly “Blue Moon” can’t occur is in February, which only lasts for 28 days (or 29 days, as in 2020, which is a leap year), so occasionally February doesn’t even have one full Moon.
When is the next ‘Blue Moon?’
The next monthly “Blue Moon” is this coming Saturday, but the next seasonal “Blue Moon” is on August 22, 2021.
The next time there will be both types of “Blue Moons” in the same year will be in 2037.
What does a ‘Blue Moon’ look like?
It looks like any other full Moon. So not blue at all. Still, that’s to underplay what the Moon looks like on the date of the full Moon. Sure, when it’s high in the sky it’s nothing special—just a bright, white orb that’s too glarey to look at.
However, as it appears, in muted oranges and yellows, on the horizon in the east at dusk, and when it drops low to the western horizon at moonset at dawn the previous and following morning, the full Moon is a phenomenal sight.
Do ‘Blue Moons’ matter?
Since a ‘Blue Moon’ isn’t blue and isn’t even different to any other full Moon, should we even be talking about it? Of course we should. All discussion of the Mon is good because it gets us outside looking up at our satellite when we may have just stayed indoors watching TV.
If a “Blue Moon” gets us to moon-gaze more—and particularly if it allows anyone to witness a beautiful moonrise or moonset for the first time—then it’s alright by me.
Wishing you clear skies and wide eyes.