A popular name, not a technical term
Astronomers do not use “blood moon” as a formal term. They speak of a total lunar eclipse, the phase when the entire Moon is inside the darkest part of Earth’s shadow, called the umbra. The nickname caught on because of the color: instead of disappearing, the Moon glows copper, rust or deep red.
The color comes from sunlight that has passed through Earth’s atmosphere and been bent into the shadow. The atmosphere strips out most blue light, so the light that reaches the Moon is reddened, like sunset light. The full physics is explained in why is the moon red.
Other meanings of blood moon
The phrase is used loosely, so it helps to know which meaning a headline has in mind:
- A totally eclipsed Moon. This is the most common modern meaning.
- A Moon that looks red near the horizon or through wildfire smoke. This is the same atmospheric filtering, without an eclipse.
- A traditional full Moon name. In some folklore lists, the October full Moon, also called the Hunter’s Moon, is occasionally called a blood moon.
- One of four eclipses in a tetrad. A tetrad is a series of four total lunar eclipses in a row, each about six months apart, with no partial eclipses in between.
The 2014–2015 tetrad
The term became widely known after a tetrad of total lunar eclipses on 15 April 2014, 8 October 2014, 4 April 2015 and 28 September 2015. Some writers attached predictions of world events to these four “blood moons”. Nothing unusual followed. Tetrads are a normal result of the Moon’s orbit and occur in some centuries more than others.
How often can you see one?
A total lunar eclipse needs a full Moon that passes almost exactly through the center line of Earth’s shadow. Because the Moon’s orbit is tilted by about 5 degrees relative to Earth’s orbit around the Sun, most full Moons pass above or below the shadow.
Worldwide, total lunar eclipses happen on average a little less than once a year, but they are irregular: some years have two, and some have none. Each one is visible from the half of Earth where the Moon is above the horizon during totality, so from any single place you can expect to see one every few years, weather permitting. Check the eclipse calendar for upcoming dates and where they are visible.
What a blood moon looks like
The eclipse unfolds slowly over several hours:
| Stage | What you see |
|---|---|
| Penumbral phase | Very faint dimming, often unnoticed |
| Partial phase | A dark, curved bite spreads across the Moon |
| Totality | The whole Moon turns red, orange or brown |
| Partial phase again | The bite retreats from the opposite side |
| End | The full Moon returns to normal brightness |
The red glow lasts only during totality, which can range from a few minutes to well over an hour. The shade varies from eclipse to eclipse: bright orange when Earth’s atmosphere is clear, and dark brick red or nearly black when it holds a lot of volcanic dust.
Blood moons in history and culture
Many cultures gave meaning to a reddened Moon. Some saw it as a bad omen, while others told stories of a creature attacking the Moon. The most famous historical account comes from 1504, when Christopher Columbus, stranded in Jamaica, used an almanac’s prediction of a lunar eclipse to persuade local people to keep supplying his crew. Today the event is simply a free and safe sky show.
How to watch
All you need is a clear view of the Moon. Find the start and end times in your own time zone on the eclipse calendar, and read how to view a lunar eclipse for tips on binoculars and photography. A blood moon always falls on the night of a full Moon, which you can confirm on the moon phases page.