A blood moon turns the lunar disk the color of dried rust. A blue moon is simply the second full moon in one calendar month. The blood moon is a total lunar eclipse, a physical event you can clock to the minute; the blue moon is a calendrical quirk, a name for timing, not for color. One happens roughly twice a year, the other about once every two and a half years, and conflating them confuses astronomy with folklore in ways that matter for anyone tracking the sky.
What Is a Blood Moon? What Is a Blue Moon?
The blood moon occurs when the Sun, Earth, and Moon align in a straight line, with Earth in the middle. The Moon passes through Earth’s shadow, and the only sunlight that reaches it bends through our atmosphere, scattering blue light and leaving red. That is why the eclipsed Moon glows copper, brick, or rust, depending on dust and cloud cover in Earth’s atmosphere at that moment. The effect has a name in astronomy: Rayleigh scattering, the same physics that makes sunsets red.
The blue moon has no physics at all. It is a name for the second full moon inside a single calendar month, which happens because the lunar cycle runs about 29.5 days while most months run 30 or 31. A month with two full moons is not rare in the way a total eclipse is rare; it is just uncommon enough that the name stuck. The Moon itself does not turn blue. The color “blue moon” describes nothing in the sky.
The two terms share a word and almost nothing else. One is a shadow event tied to orbital mechanics. The other is a scheduling accident tied to the Gregorian calendar.
Where the Two Names Come From
The blood moon’s name is old and nearly universal. The Book of Joel in the Hebrew Bible, written around the fifth century BCE, describes the Sun turning to darkness and the Moon to blood before the day of the Lord. The phrasing carried into Christian eschatology: Acts 2:20 repeats it, and medieval commentators read every total lunar eclipse as a portent of the Last Judgment. The term “blood moon” entered modern English through this prophetic tradition, not through astronomy.
The blue moon’s history is messier and more recent. The phrase “once in a blue moon” appears in English by the early 1500s, but it meant something absurd, not something calendrical. The modern definition, the second full moon in a month, dates to a 1946 article in Sky & Telescope magazine. The author, James Hugh Pruett, misread a 1937 issue of the Maine Farmers’ Almanac and declared that a blue moon was the second full moon in a calendar month. The almanac had actually used “blue moon” for a different, more complex seasonal rule. Pruett’s error stuck. The Maine almanac’s original system, which involved extra full moons in a season, is now called a seasonal blue moon, and it still appears in some almanacs today. So the most common definition of a blue moon is, by scholarly consensus, a mistake that became tradition.
The irony is that the blood moon’s name, rooted in prophecy, describes a real physical event. The blue moon’s name, rooted in a misprint, describes a calendar artifact. The sky does not care which name we use.
What Each Event Meant to the People Who Watched It
A total lunar eclipse is visible from an entire hemisphere at once, which means every culture that kept records saw the same red disk and had to explain it. The Inca read a blood moon as a jaguar attacking the Moon, and they shook spears and shouted to drive the beast off, according to Spanish chroniclers in the 1530s. The Mesopotamians, who kept eclipse records on clay tablets from the eighth century BCE, treated a blood moon as an attack on their king: the namburbi ritual texts describe substituting a surrogate king who would sit on the throne while the real king hid, so that whatever evil the eclipse portended would fall on the substitute instead.
The blue moon never carried that weight, because it was not visible as an event. A blue moon is just a full moon. There is no moment when the sky changes. So the folklore around it is thin, mostly modern, and mostly about the phrase “once in a blue moon” meaning something that almost never happens. The oldest known use of that phrase in print is from 1528, in a pamphlet by the Protestant reformers William Roy and Jerome Barlowe, who mocked the clergy for being so corrupt that a blue moon would come before they reformed. The color was the point: a blue moon was impossible, so the reform was impossible.
That is the difference in human terms. The blood moon demanded a response, a ritual, a substitute king, a shouted curse at a jaguar. The blue moon demanded nothing. It was a joke about impossibility until 1946 turned it into a calendar fact.
How the Blood Moon Became a Prophecy and the Blue Moon Became a Schedule
The blood moon’s prophetic weight did not fade with the telescope. In 2014 and 2015, a series of four consecutive total lunar eclipses, each separated by about six months, was dubbed a “tetrad” by the American pastor John Hagee. He published Four Blood Moons in 2013, arguing that the tetrad, which fell on Jewish feast days, signaled the end times. The book sold over a million copies. The tetrad itself was not rare: NASA notes that tetrads occur about every three years on average. What was unusual was that all four fell on Passover and the Feast of Tabernacles, a coincidence that had happened only a handful of times since the first century CE.
The blue moon took the opposite path. It became administrative. The 1946 Sky & Telescope definition spread into almanacs, then into newspapers, then into the phrase “once in a blue moon” applied to any rare event. By the 1980s, the second-full-moon definition was standard enough that the Old Farmer’s Almanac began using it, even though the almanac had once followed the seasonal rule. The blue moon is now a scheduling note in astronomy apps. It does not predict anything. It does not warn anyone.
The blood moon still carries the old fear. The blue moon carries a spreadsheet.
What People Get Wrong About Both
The most common error is thinking a blood moon is a separate type of moon rather than a phase of a total lunar eclipse. Every blood moon is a total lunar eclipse, and every total lunar eclipse produces a blood moon. There is no blood moon that is not an eclipse.
The second error is the color of the blue moon. The Moon can appear blue, but only after volcanic eruptions or massive wildfires, when ash particles in the atmosphere scatter red light and let blue through. The 1883 eruption of Krakatoa produced blue moons for nearly two years, and the 1950 Canadian forest fires did the same. Those are actual blue moons. The calendrical blue moon has nothing to do with color.
A third error is the frequency. People say “once in a blue moon” as if it meant almost never. A blue moon happens about every 2.7 years. A blood moon happens about twice a year, though not all are total. The phrase is a hyperbole, not a calendar.
A fourth error is the “supermoon blood moon” confusion. A supermoon is a full moon at perigee, the closest point in its orbit, which makes it look larger. A supermoon can coincide with a total lunar eclipse, which happened in September 2015 and again in January 2018. That combination is genuinely rare, but it is not a separate category of moon. It is two events stacking.
FAQ
Is a blood moon the same as a total lunar eclipse? Yes. A blood moon is the popular name for the appearance of the Moon during a total lunar eclipse, when Earth’s shadow reddens it.
How often does a blue moon occur? About every two and a half years, on average, because the 29.5-day lunar cycle fits into a 30 or 31 day month about once every 32 months.
Can the Moon actually turn blue? Yes, but only after volcanic eruptions or major wildfires, when atmospheric particles scatter red light. The calendrical blue moon never changes color.
Does a blood moon have any effect on people? No measurable physical effect. The folklore is rich, but the eclipse is a shadow event with no influence on human biology or behavior.
The Sky Keeps Its Own Calendar
The blood moon and the blue moon share a word and diverge completely. One is a shadow, a prophecy, a moment when the sky turns the color of iron left in rain. The other is a footnote in a 1946 magazine article, a scheduling accident that became a phrase. Both names survive because people need words for the sky’s irregularities, whether those irregularities are real or invented.
If you want to track the next one, the eclipses guide covers the mechanics of shadow events in detail. For the symbolism that grew around lunar cycles, see the glossary entry on lunar phases, and for the prophetic tradition that turned eclipses into omens, read the guide to eclipse prophecy. The sky does not care what we call it. We name it anyway, because naming is how we make the dark legible.