Once in a blue moon, there is a Blue Moon. Don’t look too hard, the moon doesn’t actually turn blue. It is just a name we give to it because it doesn’t happen very often. As it turns out there are two types of blue moons.
- A seasonal blue moon is the third full moon within a three month season that has four full moons.
- A monthly blue moon is the second full moon within a calendar month.
A blue moon was initially defined as just a seasonal blue moon. This definition might have come from a 1937 article in the Maine Farmers’ Almanac. However, in March 1946, James Hugh Pruett described it as a monthly blue moon in Sky & Telescope. Over time, the monthly definition has stayed with us.
So, get ready, the next monthly blue moon is coming up on Sunday, May 31, 2026. Although, you will have to wait until Thursday, May 20, 2027 for the next seasonal blue moon.
By the way, there is a red moon (more commonly called a blood moon). This occurs during a lunar eclipse when the Sun’s light is blocked by the Earth and only a small portion sneaks through the Earth’s atmosphere giving the Moon a reddish color.
- Vigdis Hocken and Aparna Kher. “What Is a Blue Moon and When Is the Next One?” TimeandDate.
- “What is a blue moon? Is the moon ever really blue?” Library of Congress.
- “What is a blue moon?” Royal Museums Greenwich.
- “What Is A Blue Moon And When Is The Next One?” Famers Almanac.
- Doris Elin Urrutia. “Why is the idea of a Blue Moon so enchanting?” space.com. May 30, 2026.
- Jess Wilder. “Blue moons explained.” BBC Sky at Night Magazine. May 20, 2026.
- Maggie Ekberg. “The Real Reason We Say “Once in a Blue Moon”—And When the Next One Is Coming.” Readers Digest. May 15, 2026.
- Bruce McClure and Deborah Byrd. “Full Blue Moon – and farthest moon of 2026 – on May 30-31.” EarthSky. January 1, 2026.
- PHILIP HISCOCK. “WHERE DOES THE PHRASE “ONCE IN A BLUE MOON” COME FROM?” Sky & Telescope. August 24, 2012.
- “DONALD W. OLSON, RICHARD TRESCH FIENBERG, AND ROGER SINNOTT. WHAT IS A BLUE MOON IN ASTRONOMY?” Sky & Telescope. July 27, 2006.
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