Your turn
Each lab is one part of the video. Change things and see what happens.
Each lab isolates one step. Facts from NASA and OpenStax; the pictures are not to scale.
Tap the buttons and slide the sliders.
Slide through the night. When does the bite appear, and when does the Moon turn red?
Move the Moon through penumbra, partial and total phases. Note how faint the penumbral stage is.
Slide from evening to morning. When does the Moon turn red?
Hours of eclipse squeezed into one slider.
Drag the Moon up and down. Where must it be for a total eclipse?
Most full Moons pass above or below the umbra. Find the band where the eclipse is partial, and where it is total.
Slide the Moon into Earth’s shadow. What happens?
Not to scale: the real Moon is about 30 Earth-widths away.
Turn Earth’s air off and on. What does the eclipsed Moon look like without it?
Without an atmosphere no sunlight is refracted into the umbra. Compare the two cases.
Tap the button. Does the Moon go dark?
The air layer is drawn far thicker than it really is.
Slide how much air the sunlight crosses. What colour gets through?
A longer path through air scatters more short-wavelength light out of the beam. Watch the transmitted colour shift.
Slide to add more air. Does the light turn red?
A simplified colour ramp, not a measured spectrum.
Move the Moon, add volcanic dust or clouds. Which shade do you get?
Dust, aerosols and clouds along Earth’s limb darken totality. Estimate the Danjon value for each setting.
A toy model of how dust and clouds dim the Moon.
Move the Moon up and down the sky, and add smoke. When does it look orange?
Near the horizon moonlight crosses much more air. Smoke particles filter blue and green even more strongly.
Slide the Moon up and down. When is it orange?
The air bar is a rough relative measure.
Sometimes the big round Moon turns red. People call it a blood Moon. It is not scary at all!
The Moon has no light of its own. The Sun shines on it. Our Earth makes a big shadow, just like you do on a sunny day. Sometimes the Moon slides right into that shadow.
Earth has a thin blanket of air. The air bends a little sunlight into the shadow, like a straw looks bent in a glass of water. That is why the Moon does not go dark.
Sunlight has all the colours mixed together. When light goes through lots of air, the blue gets bumped away. Only red and orange are left, like at sunset. That red light lands on the Moon.
A Moon near the ground can look orange too. Its light goes through lots of air. Smoke from a big fire can make it red as well.
A deep red Moon, often called a blood Moon, is a total lunar eclipse. Earth sits exactly between the Sun and the full Moon, so the Moon slides into Earth's shadow.1 You might expect it to vanish. Instead it glows a dull coppery red, because Earth's atmosphere bends a little sunlight into the shadow.5 On its way through the air, that light loses most of its blue, so mostly red and orange light reaches the Moon.12
NASA puts it beautifully: it's as if all the world's sunrises and sunsets are projected onto the Moon at once.2
If there's no eclipse, a red or orange Moon is almost always either low near the horizon or seen through smoke or dust. It's the same trick: light crossing a lot of air comes out red.89 (There's a section on that below.)
Without an atmosphere, the eclipsed Moon would go almost completely dark. The same thing that paints our sunsets is what keeps it glowing.4
Sunlight looks white, but it's a mix of every colour of the rainbow. Tiny air molecules scatter blue light more than other colours, because it travels as shorter, smaller waves. That scattered blue light, bouncing around overhead, is why the sky is blue.7 (Our explainer Why is the sky blue? lets you play with this.)
When the Sun is low, its light takes a long, low path through the air to reach you. Most of the blue is scattered away in the distance, and only the yellow-to-red part arrives.29 The light that reaches an eclipsed Moon has done the same thing, skimming through a thick slice of air all around Earth's edge.2
more air crossed → less blue → redder light
If you stood on the Moon during a total lunar eclipse, you'd see Earth as a black disc ringed with a thin, glowing red line: every sunrise and sunset on the planet at that moment.10
Earth's shadow has two parts, and which part the Moon crosses decides what you see.1
| Type | Where the Moon goes | What it looks like |
|---|---|---|
| Penumbral | Only through the faint outer shadow (penumbra) | A slight dimming, easy to miss |
| Partial | Partly into the dark inner shadow (umbra) | A dark "bite" grows and shrinks, never covering the whole Moon |
| Total ("blood Moon") | Completely inside the umbra | The whole Moon glows red, orange or coppery |
During a total eclipse, the Moon dims slightly for about 20 minutes as it enters the penumbra. Then a curved dark bite appears: the edge of Earth's round shadow.52 While the Moon is only partly in the umbra, the shadowed part looks very dark next to the bright rest of the Moon. The red shows best once the whole Moon is inside.2
Earth is about four times wider than the Moon, so its shadow is big.6 At the Moon's distance, the umbra could cover about four full Moons.5 When the Moon passes close to the centre of the shadow, the red, total part can last up to about 1 hour and 40 minutes, and each partial phase before and after takes at least an hour.5 Counting the faint penumbral stages, the Moon can be in Earth's shadow for up to about five hours.6
The Moon's orbit is tilted by about 5 degrees compared with Earth's path around the Sun. At most full Moons, the Moon passes a little above or below Earth's shadow and misses it.16 Only when a full Moon happens near a crossing point of the two paths do we get an eclipse. Our explainer Why does the Moon have phases? shows why the phases themselves are not caused by Earth's shadow.
Lunar eclipses of some kind happen about two to five times a year, and a total lunar eclipse happens at least twice every three years.6 Each lunar eclipse can be seen from about half of Earth: the whole night side, wherever the Moon is up.15 That's why many more people see lunar eclipses than total solar eclipses, whose dark shadow falls on a narrow strip. From any one place, though, total lunar eclipses come less often.6 For the other kind of eclipse, see What causes a solar eclipse?
No two total eclipses look quite the same. The colour depends on what's in Earth's air at that moment, all around the edge of the planet: dust, water droplets, clouds and mist.104
Astronomers rate an eclipse's brightness on the Danjon scale, judged near the middle of totality:4
| L | What the Moon looks like |
|---|---|
| 0 | Very dark; the Moon is almost invisible |
| 1 | Dark, grey or brownish; details hard to see |
| 2 | Deep red or rust-coloured, darker in the middle |
| 3 | Brick red, often with a bright or yellowish rim |
| 4 | Very bright copper-red or orange, with a bright, bluish rim |
Keen watchers sometimes spot a faint turquoise band near the shadow's edge at the start or end of totality, which is thought to come from the ozone layer.10
Many red Moons have nothing to do with an eclipse. Two everyday causes are much more common:
So the quick test is: is it a full Moon, and has an eclipse been announced? If yes, it's probably a blood Moon. If not, look at how low the Moon is and whether the air is hazy.
Yes. A lunar eclipse is completely safe to watch with your bare eyes. You don't need special glasses. Binoculars or a small telescope make the colour even richer, and a dark spot away from bright lights helps.32 That's the big difference from a solar eclipse, where looking at the Sun without proper eclipse glasses can hurt your eyes.3
The name "blood Moon" is just a popular nickname, not an official astronomy term.6 Over history, people have given red Moons all sorts of meanings, but eclipses are regular, predictable events and completely harmless.
If there's a total lunar eclipse, the Moon is inside Earth's shadow and lit only by red sunlight bent through Earth's air. On a normal night, a red or orange Moon is usually just low near the horizon, or seen through smoke or dust. Its light crosses so much air that the blue is scattered away.
A blood moon is a total lunar eclipse. Earth sits exactly between the Sun and the full Moon, so the Moon passes through Earth's dark shadow. Earth's atmosphere bends some sunlight into the shadow, and on the way it scatters out the blue, so only red and orange light reaches the Moon.
The red, total part can last up to about 1 hour and 40 minutes. Counting the partial phases before and after, the Moon can be in Earth's shadow for several hours.
Scientifically, it means the moonlight you see (or the sunlight reaching the Moon) has passed through a lot of air, dust or smoke, which filters out blue light. People have attached many meanings to red Moons over the centuries, but eclipses are regular, predictable events and completely harmless.
Yes. Unlike a solar eclipse, a lunar eclipse is safe to watch with your bare eyes. You don't need special glasses. Binoculars make the colour even clearer.
Lunar eclipses of some kind happen about two to five times a year, and a total lunar eclipse happens at least twice every three years. Each one can be seen from the whole night side of Earth, but from one spot you'll see a total one less often.
The Moon's orbit is tilted by about 5 degrees compared with Earth's path around the Sun. At most full Moons, the Moon passes just above or below Earth's shadow and misses it.