A once-in-a-Blue Moon event is happening this weekend – literally. Stargazers who step outside and look up this weekend will be able to see a “Blue Moon” above them. But what does that mean?
What is a Moon?
Moon phenomena do have some wacky names – super blood wolf moon, anybody? – but unfortunately, “blue” isn’t actually one of them. The term is not astronomical but folklore, and is understood to have two meanings: It can either refer to the second full moon in a calendar month, like last year's Hallowe’en Blue Moon, or, like this weekend, the third full moon of the season. There’s nothing scientific between the discrepancy in these definitions – it just comes down to differing traditions.
This Blue Moon is a result of an unusually timed summer (or winter, for those below the equator). That actually is a scientific term: in this case, it means the period of time between the June solstice and the September equinox. Normally, the seasons that span the times between the solstices and equinoxes last three months, and thus we see three full moons. But this year, things were a little different: the June full moon happened to occur just three days after the solstice, leaving enough summertime for four full moons overall. And no, we don’t know why it’s the third moon, rather than the first or the fourth, that gets the special name. Folklore is weird.
When to see the Blue Moon
The best time to see the Blue Moon is moonrise. There are two successive nights that the full Blue Moon will be visible: Saturday, August 21, and Sunday, August 22. The Moon will actually be full at 12:02 am UTC on Sunday but you can check the precise time for moonrise in your region here.
How rare are Blue Moons?
So how rare is a once-in-a-blue-moon event, anyway? Some quick math courtesy of EarthSky can help us out here: a seasonal Blue Moon like Sunday’s occurs seven times every 19 years. That’s a strangely specific amount, we know, but it’s because of the difference between a lunar month and a calendar month. A lunar month is a bit shorter than a calendar month on average, which means that the two don’t sync up very well most of the time – but every 19 years, the phases of the Moon repeat themselves on (or close to) the same calendar dates. Now, 19 years is 235 lunar months, each with its own full moon, but it’s only 228 calendar months – leaving seven “extra” full moons over the full period.
Is the Moon actually Blue?
Sadly, it’s highly unlikely that the Moon will actually be blue – although that can happen. The Moon can appear blue in color when the atmosphere is heavy with certain red light-scattering particles. This can happen to any moon, not just when it's full. These are particularly prevalent around wildfires, though, so places like British Columbia or California might be graced with a romantic cerulean orb to light up their local hellscape.
If you miss this weekend’s Blue Moon, you’ll have to wait until August 2024 for the next one – or, if you’re impatient, you can settle for the “second of two moons in one month” version which will turn up a year earlier. Either way, it’s a pretty neat phenomenon, so keep your eyes peeled – and remember, the Sturgeon Moon rising in Leo in the realm of Aquarius means … well, nothing. It’s our planet's natural satellite, not a magic guidance counselor.