What A Blue Moon Actually Is? [Blue Moons Aren't Blue] #astronomy #didyouknow #space

Added: 2026-05-23

[00:00:00]Once in a blue moon, >> [music] >> you've said it. But what's actually a blue moon? It's not blue.

[00:00:05]There are two definitions.

[00:00:07]The original one is called a seasonal blue moon. Most seasons have three full moons.

[00:00:13]Every so often you get a season with four.

[00:00:15]The third one, not the fourth, the third, is the blue moon.

[00:00:20]That's how the phrase started, but in 1946, a magazine got the definition [music] wrong. They wrote that a blue moon was the second full moon in a single calendar month.

[00:00:30]That mistake stuck, and now most articles use the wrong definition.

[00:00:35]Both happen rarely. The seasonal kind, [music] roughly every two to three years. The monthly kind, about every two and a half years.

[00:00:43]Neither makes [music] the moon actually blue.

[00:00:46]The only time the moon really turns blue is when there's a lot of ash or smoke high in the atmosphere, from a volcanic eruption or a massive forest fire.

[00:00:54]That's where the phrase actually comes from.

[00:00:57]Volcanic eruptions [music] can turn the moon blue, but only once in a blue moon.

[00:01:01]The next blue moon by the monthly definition is May 31st, 2026.

[00:01:07]If you remember a particular night clearly, maybe the moon was full, maybe it wasn't. You can see exactly which moon phase was overhead at sky-when.com.

[00:01:16]The exact moon, the exact [music] night, sky-when.com.