What is a Blue Moon? When’s the next one?

Blue Moons happen about once every 33 months and, no, the Moon doesn't turn blue.
By | Published: January 15, 2024

Blue Moons are known for the folklore surrounding their appearance in the skies. But the real thing has less to do with its color and more with the timing of the year’s Full Moons.

The next Blue Moon is Aug, 19, 2024. It is this year’s only one and is a “seasonal” one. Here’s why. 

There are two types of Blue Moons: seasonal and monthly. A seasonal one is “the third Full Moon of an astronomical season that has four Full Moons” and happens between the summer solstice and the fall equinox, according to NASA. A monthly Blue Moon is the second Full Moon in a month that features two Full Moons.

The next monthly Blue Moon is May 31, 2026. The last was Aug. 31, 2023. By the way, it’s possible that those who live in certain time zones may not experience a Blue Moon. Here’s how Astronomy Magazine explained it in 2004: “Astronomers calculate the instant of Full Moon using Universal Time (UT), a standard that is, for our purposes here, equivalent to Greenwich Mean Time. If you happen to live in a time zone far enough away from Greenwich, England, shifting the UT time of Full Moon to your local time may be enough to advance the date to the first day of the new month, which means it’s no longer a Blue Moon. Depending on your time zone, the Blue Moon of July 2004 occurred in either July or a month later, with the second Full Moon of August.”

How rare are they?

The idiom suggests a rare event that happens after a long time or never at all, but they occur about once every 33 months. Monthly Blue Moons happen when there is one Full Moon at the start of a calendar month, so there are at least 29.5 days between the two Moons. Either type will occur about every two or three years. Every two and a half years, there is a 13th Full Moon in a calendar year. This happens because Moon phases occur every 29.5 days or 354 days for 12 Full Moon cycles. Because this is less than 365 days, there is a Blue Moon around every two and a half years.

Between 1550 and 2650, 408 seasonal Moons and 456 monthly Blue Moons occurred or will occur. Both seasonal and monthly can happen in the same year. Almanacs first began to refer to the extra full Moons as Blue, according to the Encyclopedia Britannica. An error in a magazine “that the extra moon was the second in a calendar month rather than the fourth in a season, [provided] a new meaning that eventually crept into wide usage in the 1980s.”

Does the Moon turn blue?

No, the moon does not turn blue. However, atmospheric conditions like smoke or volcanic ash can make the moon appear indigo.