Blue moons aren’t actually blue, but they are as rare as the saying goes.
The second full moon in a month is commonly referred to as a blue moon—we had a full moon on July 2, and we’ll have another this Friday on July 31—but there’s a more complicated definition according to the U.S. Naval Observatory:
“The traditional definition is that of the third Full Moon to occur in an astronomical season in which four Full Moons fall. While somewhat more cryptic than the ‘second-of-the-month’ variety, this scheme helped farmers and other people who used traditional Full Moon names to keep the proper names in synch with the occasional “extra” Full Moon that occurs about every 2.6 years. The “controversy” over the proper definition of the “Blue Moon” dates back to an erroneous discussion of the phenomenon that was originally published in 1946 and subsequently re-discovered in 1999.”
More Must-Reads from TIME
- Cybersecurity Experts Are Sounding the Alarm on DOGE
- Meet the 2025 Women of the Year
- The Harsh Truth About Disability Inclusion
- Why Do More Young Adults Have Cancer?
- Colman Domingo Leads With Radical Love
- How to Get Better at Doing Things Alone
- Michelle Zauner Stares Down the Darkness
Write to Nolan Feeney at nolan.feeney@time.com