Blue Moon Myths and Facts – What Everyone Gets Wrong

What Is a Blue Moon?

Here’s something most people don’t know: there are actually two completely different definitions of a blue moon, and they’ve been confusing people for decades.

The Modern Definition

The version most of us know today says a blue moon is the second full moon in a single calendar month. Since the average time between full moons is 29.5 days, two can occasionally squeeze into the same month, as long as the first one falls early enough. This happens roughly every two to three years, which is probably where we get the phrase “once in a blue moon.” Not impossibly rare, but definitely not common.

The Original Definition

The older definition, which comes from early editions of the Maine Farmer’s Almanac, defines a blue moon as the third full moon in a season that has four full moons instead of the usual three. Most years have 12 full moons. Every now and then, there are 13. That extra moon threw off the old ecclesiastical calendar, which had specific names and purposes for each moon of the year. By calling the extra one a “blue moon,” the calendar stayed on track. It’s a more complex definition, but it’s the original.

Does It Actually Turn Blue?

Almost never. The moon doesn’t change color just because there are two in a month. The only time the moon can genuinely appear blue is when smoke or volcanic ash particles of a specific size are present in the atmosphere. It’s happened after major volcanic eruptions, but it’s genuinely rare. Most photos you see of a “blue moon” are using filters or photo editing.

Recent and Upcoming Blue Moons

We just had one! The most recent blue moon occurred on May 30-31, 2026, the second full moon in May. It was also 2026’s smallest full moon, sitting about 252,360 miles from Earth, making it roughly 7% dimmer than average. EarthSky

The next seasonal blue moon takes place on May 20, 2027, and the next monthly blue moon after that is December 31, 2028. Time and Date

Looking further out: seasonal blue moons are also coming on August 24, 2029, August 21, 2032, and May 22, 2035. EarthSky

The Double Blue Moon Phenomenon

Very rarely, both a seasonal and a monthly blue moon occur in the same calendar year. This will next happen in 2048, when a monthly blue moon falls on January 31 and a seasonal blue moon on August 23. Star Walk

Where the Phrase Comes From

The expression “once in a blue moon” is older than most people realize. The earliest recorded reference dates to 1528, when saying the moon was blue meant something was obviously absurd, basically impossible. Over the centuries the meaning softened into what we use today: something that happens rarely, but does eventually happen. Whether the saying got tied to the actual lunar event or developed separately is still debated.

Tags: blue moon, Moon