A Blue Moon is a rare sight. And, most of the time, it isn’t actually the color blue. A Blue Moon occurs when there are two Full Moons in one month. Full Moons happen about every 29 days. Most months have 30 or 31 days, so occasionally, a month will have two Full Moons. On average, Blue Moons happen once every two-and-a-half years.
Even more rarely, the moon does appear to take on a blue hue. What’s going on? Is the moon full of color-changing LEDs? Is it actually a big disco ball in the sky that reflects the colors on Earth? No, of course not!
When the moon appears orange, red, or blue, it has nothing to do with the moon itself changing color. Instead, it’s made to look blue by particles in Earth’s atmosphere. For example, the moon did suddenly appear blue to people in Indonesia in 1883.
That year, the volcano Krakatoa erupted. It was an enormous explosion. In fact, some experts compare it to a nuclear bomb. The volcano spewed ash into the air, which stayed in the atmosphere for quite a while. It had the effect of making the moon appear blue. In fact, on some nights, the moon looked green. During the day, the sun even appeared to be lavender!
Of course, “Blue Moon” also has another meaning. Have you ever heard the phrase “Once in a Blue Moon”? When people say that, they mean that something happens very rarely—like a Blue Moon!
“Once in a Blue Moon” is a common idiom. That means it’s a phrase that means something other than what the words mean when you read them literally. When people say “Once in a Blue Moon,” they don’t mean once every two-and-a-half years. Instead, they mean “rarely” or “not often.”
When is the next Blue Moon? Watch the sky on the night of October 31, 2020. That’s when you’ll see the second Full Moon of that month rise in the sky—a Blue Moon! There will also be a Blue Moon in August 2023 and May 2026.