A blood moon is usually a total lunar eclipse. Earth passes directly between the Sun and the full Moon, and the only sunlight that reaches the Moon has been bent and filtered through Earth's atmosphere, which turns it a deep copper or rust red. "Blood moon" isn't an astronomical term, and people also use it loosely for a full moon that looks red near the horizon, or as an old name for October's full moon.
Why the Moon turns red
During a total lunar eclipse, the Moon sits inside the darkest part of Earth's shadow, the umbra. You might expect it to vanish completely. It doesn't, because Earth's atmosphere acts like a lens. Sunlight grazing the edge of the planet is bent inward into the shadow, and on the way through the air most of the blue light is scattered out, the same process that makes the daytime sky blue. What's left is the red and orange end of the spectrum.
A good way to picture it: if you stood on the Moon during totality, you would see Earth as a black disc ringed with fire, every sunrise and sunset on the planet happening at once. That ring is what lights the Moon.
The exact color changes from eclipse to eclipse, depending on how much dust, cloud and volcanic haze is in Earth's atmosphere at the time. After a big volcanic eruption, an eclipsed Moon can turn so dark it almost disappears. In clean air it can glow a bright orange. Astronomers grade this on the Danjon scale, from 0, a nearly invisible Moon, to 4, a bright copper or orange one, and amateur observers send in their estimates after every eclipse.
Three things people mean by "blood moon"
The phrase gets used in a few ways, which is why searches turn up such different answers:
- A total lunar eclipse. The modern, most common meaning, and the one news stories use.
- A full moon low on the horizon. Any rising full moon can look orange or reddish through the thick air near the ground. This is lovely, but it's ordinary and happens every month.
- A name for the October full moon. Some almanac lists give Blood Moon or Sanguine Moon as an alternative name for the Hunter's Moon, linked to the hunting season. See Hunter's Moon meaning for that one.
When a headline says "blood moon tonight," it almost always means the first.
A blood moon in old stories
Before anyone could explain them, eclipses were frightening, and a Moon turning the color of blood was an obvious omen. In ancient Mesopotamia, scholars kept careful records of eclipses and read them as messages about the king. During some eclipses considered especially threatening, royal advisers seated a temporary substitute on the throne so that whatever misfortune was coming would fall on him instead.
The Hebrew Bible's book of Joel speaks of the Sun turned to darkness and "the moon into blood" before a great day, a line quoted again in the New Testament book of Acts. Those verses are the source of much of the modern apocalyptic interest in eclipses. In the mid-2010s, a run of four total lunar eclipses in a row, called a tetrad, drew a wave of end-of-the-world predictions. The eclipses came and went on schedule, as eclipses do.
One famous story runs the other way. In 1504, stranded on Jamaica and short of food, Christopher Columbus used his almanac's prediction of a lunar eclipse to persuade local Taíno people to keep supplying his crew. NASA's catalog of historical eclipses includes it, with the note that listing an event isn't proof of the story's details.
What astrologers make of a blood moon
In astrology, a total lunar eclipse is a full moon with extra weight. Every lunar eclipse is a full moon that falls close to one of the lunar nodes, the two points where the Moon's path crosses the Sun's. Astrologers read full moons as moments of culmination: something comes to a head, comes to light, or is ready to be let go. An eclipse, in that view, makes the same themes sharper and less comfortable to ignore.
The sign matters. A blood moon in Leo, with the Sun in Aquarius, points at the pull between personal pride and the group. One in Pisces, with the Sun in Virgo, sets the details against the bigger picture. Astrologers also look at which house of your chart the eclipse falls in, and whether it lands close to one of your own planets. We go through that step by step in lunar eclipse astrology.
Treat it as an invitation to reflect, not a warning. Nothing about an eclipse decides how your week goes, and the old idea of eclipses as bad omens says more about how frightening they were before anyone could predict them.
How to watch one
Total lunar eclipses are the friendliest of sky events:
- No eye protection needed. Unlike a solar eclipse, a lunar eclipse is safe to watch with your bare eyes, binoculars or a telescope.
- Half the world can see it. Anyone on the night side of Earth with the Moon above the horizon gets the same view at the same moment, local clock time aside.
- It takes hours. The partial phases, as Earth's shadow takes its first bite and then gradually covers the Moon, can each take about an hour. Totality itself can last anything from a few minutes to about an hour and three quarters.
- Binoculars help. They bring out the color gradients across the Moon's face, often darker on one edge than the other.
- For photographs, use a tripod and expect to lengthen your exposure as the Moon darkens.
Our full moon calendar lists the next twelve full moons and flags the ones that are eclipses. If you want to mark the night, a few minutes of quiet with a notebook, or the full moon ritual guide, is plenty.
Questions readers ask
Is a blood moon bad luck?
No. It's a predictable alignment of the Sun, Earth and Moon that astronomers can calculate centuries in advance. Many cultures once read eclipses as omens, and some astrologers still treat them as intense times, but nothing about a red Moon causes misfortune.
How often is there a blood moon?
Total lunar eclipses happen on average a little less than once a year somewhere on Earth, though they come irregularly: sometimes two in a year, sometimes none for a couple of years. Any one location sees only some of them, depending on whether the Moon is up at the time.
What is a super blood moon?
A total lunar eclipse that happens when the Moon is near perigee, its closest point to Earth. It looks slightly larger than other eclipsed moons. The labels are independent, which we explain in what is a supermoon.
What does The Moon tarot card have to do with it?
Only by theme. The Moon card is about the half-lit world of instinct, dreams and things that aren't what they seem, which suits a night when the familiar full moon turns strange. You can read the full meaning on The Moon card page.
