1. Why the Moon Does Not Go Dark Inside Earth's Shadow
Earth's shadow has a very dark inner part, called the umbra. In a total lunar eclipse the Moon moves completely into it[1]. If Earth were a ball with no air around it, no sunlight could reach the Moon there, and it would look black.
Earth does have air, though. A ring of atmosphere surrounds the planet, and some sunlight passes through it before it ever gets to the Moon. That is the light we are looking for.
2. Earth's Atmosphere Bends Sunlight Into the Shadow
Japan's National Astronomical Observatory (NAOJ) explains that the atmosphere works a little like a lens: sunlight that grazes the edge of Earth is bent, and some of it ends up inside the umbra[2]. Think of looking at a pencil in a glass of water. The pencil seems to bend where light crosses from one material to another. Light crossing the edge of the atmosphere bends in a similar way, though far more gently.
So the Moon in total eclipse is lit, but only by a thin leftover glow that came around the planet. It is dim, which is why the Moon looks so much darker than usual. Why is the leftover glow red rather than white?
3. The Air Scatters Away Blue Light and Lets Red Light Through
Sunlight is a mix of colors. As it travels through air, short-wavelength blue light is scattered in all directions much more than long-wavelength red light. Little blue light makes it through, while red light is weakened but gets through[2][1]. This is the same reason sunrises and sunsets are red: at those times sunlight slants through a long stretch of air.
NASA describes it in a similar way: the light reaching the Moon in an eclipse has passed through Earth's atmosphere, a bit like sunrises and sunsets from all over the world shining onto the Moon[1]. Picture standing on the Moon and looking back at Earth. You would see a dark disc with a thin glowing ring around it, as if sunrises and sunsets all over the world were happening at once. This is a picture to help you imagine it, not a photograph, but it matches the explanation above.
4. Why the Moon's Color Is Different From One Eclipse to the Next
The red is not the same every time. NAOJ says that when the air holds little dust the Moon looks bright orange, and when it holds a lot the Moon can look dark, nearly black[3]. NASA's Danjon page likewise says that dust, clouds, and volcanic ash in the atmosphere filter and dim the sunlight that reaches the umbra[4].
What both say is that the state of the air matters: the brightness and color of the eclipsed Moon change with what is floating in it.
NAOJ gives two famous examples, both linked to volcanoes[3]:
- December 30, 1982. The Moon was almost invisible during totality. This is thought to be due to the eruption of El Chichón in Mexico that spring, which is believed to have sent ash and gases high into the stratosphere.
- June 4, 1993. Most of the Moon looked gray, with only a slight red tint. This is presumed to be the lingering effect of the huge eruption of Mount Pinatubo in the Philippines in June 1991, about two years earlier.
Notice the careful words "thought" and "presumed." Nobody can go back and test an eclipse that has already happened, so the link between volcano and dark Moon is the best explanation, not something proven. Still, it is striking that a distant eruption could change what the Moon looks like to people on the other side of the planet.
5. The Danjon Scale: A Five-Step Way to Rate the Moon's Color
Since the color changes, astronomers needed a simple way to write it down. The French astronomer André-Louis Danjon proposed a five-point rating from 0 to 4, now called the Danjon scale. NASA's eclipse pages describe it like this[4]:
- 0: very dark eclipse, the Moon almost invisible, especially at mid-totality
- 1: dark, gray or brownish, with details hard to make out
- 2: deep red or rust-colored, with a very dark center and a fairly bright outer edge
- 3: brick-red, usually with a bright or yellow rim
- 4: very bright copper-red or orange, with a bluish, very bright rim
The rating is best made near the middle of totality (the time when the Moon is fully inside Earth's shadow), with the naked eye, binoculars, or a small telescope[4]. It is a judgment by the observer, so two people may not give exactly the same number, and the view through binoculars may differ from the naked-eye view. Judging from the descriptions, the almost invisible Moon of 1982 sits at the dark end of the scale (0), and the mostly gray Moon of 1993 sits near 1. That matching is the writer's own reading, not a rating given by NAOJ.
6. How to Watch the Next Total Lunar Eclipse and Rate Its Color
Unlike a solar eclipse, a lunar eclipse is safe to look at with your eyes, because you are only seeing moonlight. Here is what you can do:
- Find out when the next total lunar eclipse is visible where you live. NASA's eclipse tables list totality of 1 hour 42 minutes on June 26, 2029 (visible from the Americas, Europe, Africa, and the Middle East) and 54 minutes on December 20, 2029 (Americas, Europe, Africa, and Asia). There is no total lunar eclipse in 2027 or 2030[5]. Weather and your location decide what you will really see.
- Pick a time near the middle of totality, look at the Moon, and give it a number from 0 to 4. Note whether the edge looks bluish or yellowish.
- Try binoculars too. They make the Moon look brighter, so your rating may change. Write down which one you used.
- Compare your number with a friend's. If you differ, you have just seen why a standard scale can be useful.
If you want to dig deeper, the sources below are a good start, especially NASA's Danjon scale page and the NAOJ pages on eclipse color.