Why Doesn't the Moon Vanish in a Total Lunar Eclipse? It Turns Red
― A Ring of Countless "Sunsets" Around Earth Lights It
On the night of a total lunar eclipse, the Moon slips fully into Earth's shadow. No sunlight should reach it at all. Yet the Moon doesn't go black. It keeps glowing a dim coppery red. How can it shine when it is completely hidden in Earth's shadow?
On a night with a full Moon, the Moon slowly loses its edge and then slides fully into Earth's shadow. This is the start of totality. The Sun, Earth and Moon line up, and Earth blocks the sunlight. You would expect no light to reach the Moon, and the Moon to vanish from the sky.
But in fact the Moon does not go dark. It stays there, tinted "shakudō-iro" (coppery), a dark orange to red. Where does this red light come from?
The red needs two things to happen together
Because Earth has an atmosphere, its shadow is not perfectly dark. Sunlight grazing the edge of Earth is bent by the air, and is thought to reach the middle of the shadow.
Light that travels a long, slanting path through the atmosphere loses almost all its blue and keeps only the red. This is thought to be the same mechanism that makes a sunset look red.
Only when both happen together do we get a "Moon that glows red inside Earth's shadow." Let's take them one at a time.
Reason 1: Earth's shadow is not total darkness
If Earth had no atmosphere, its shadow would be a pitch-black region where no light arrives. But Earth is wrapped in a thick layer of air, and that changes things.
Some sunlight passes just past the edge of Earth. As it travels through the air, its direction bends slightly (this is called refraction). Thanks to this bending, part of the light that would have gone into the dark space behind Earth reaches the middle of the shadow instead. During an eclipse, the Moon is lit by exactly this light.
Reason 2: Light that travels far through air turns red
Look up at the sky. The blue of the daytime sky and the red of the evening sky come from the same property of air. Air molecules scatter short-wavelength blue light more strongly, sending it off in all directions. This spreading-out is called scattering. Scattering by tiny particles such as air molecules is called Rayleigh scattering (see our article on why the sky is blue for details).
During the day, sunlight crosses only a short stretch of air, so although some blue is scattered, plenty still reaches your eyes. In the evening, sunlight comes in at a slant and crosses a long stretch of air. Almost all the blue is scattered out of the beam, and only the red light, which scatters less easily, is left to reach you. That is a sunset.
The light that lights the Moon in a total eclipse also skims a long way along the edge of Earth. So this light is sometimes described as the combined glow of countless sunsets and sunrises happening all around the world at once. That is why the Moon glows red instead of going black.
The Moon in totality can look bright orange or a dark reddish brown. This is thought to be because the amount and colour of light reaching the shadow depend on the state of Earth's atmosphere at the time, such as how widespread the clouds are and how much dust volcanic eruptions have put into the air.
Something you can check yourself
- On a clear afternoon, look at the western sky from the same spot several times until sunset.
- As the Sun sinks, watch the sky around it change from whitish yellow to orange to red.
The lower the Sun, the longer the path its light takes through the air. More blue is lost along the way, and the red that remains gets deeper. In a total lunar eclipse, this same effect, "light that travels a long way through air turns red," is happening at once all around the rim of Earth. Thinking of it this way makes it easier to grasp.
Summary
The Moon looks red in a total lunar eclipse because two things happen at once. (1) Earth's atmosphere bends a little sunlight into the shadow, and (2) on its long trip through the air, that light loses its blue and keeps only the red. Together, they let the Moon keep shining inside Earth's shadow.
What lights the eclipsed Moon is the afterglow of sunsets somewhere on Earth.
The Moon in the shadow is not in darkness. It is bathed in the light of distant sunsets.
Some oddities of how we see the Moon are down to the brain, not the light. The "Moon illusion", where a newly risen Moon looks huge but doesn't in photos, is explained in this article. And the daily change in the Moon's shape, often confused with eclipses, is covered in Why does the Moon's shape change every day?, which explains that it has nothing to do with Earth's shadow.
For the curious ― terms, formulas and links to textbooksFrom middle-school science to active research, each part is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school basic Earth science or basic physics
- High school+High-school Earth science or physics, or advanced and sidebar material in textbooks
- UniversityUniversity-level specialist content (atmospheric optics), not taught in high school
- ResearchTopics researchers are still working on, not taught even at university as settled
Middle schoolTerms: words about eclipses
- Total lunar eclipse: when the Moon moves fully into Earth's shadow.
- Refraction: the bending of light's path as it passes through air of changing density.
- Scattering: light hitting small particles and spreading out in many directions.
- Rayleigh scattering: scattering by particles smaller than the wavelength of light, such as air molecules. The shorter the wavelength, the more strongly blue light is scattered.
- Shakudō-iro (coppery): the word for the Moon's colour in a total lunar eclipse. It ranges from dark orange to red.
Middle schoolHigh schoolChecking with a formula: how much bigger is Earth's shadow than the Moon?
Hearing that the Moon "fits inside the shadow" doesn't tell you whether there is room to spare. A calculation gives a clear number.
Shadow-to-Moon ratio = size of Earth's shadow ÷ Moon's diameter
| Size of Earth's shadow | About 9200 [km] at the Moon's orbit |
| Moon's diameter | About 3474 [km] |
Earth's shadow is thought to be about this wide at the Moon's distance. Comparing it with the Moon's diameter shows how much room there is.
| Shadow size as a multiple of the Moon's diameter | 9200 ÷ 3474 ≒ 2.6 [times] |
| Radius of the shadow | 9200 ÷ 2 ＝ 4600 [km] |
| Radius of the Moon | 3474 ÷ 2 ＝ 1737 [km] |
| Room on one side (shadow radius − Moon radius) | 4600 − 1737 ＝ 2863 [km] |
Earth's shadow is wide enough to hold about 2.6 Moons side by side. The Moon does not just barely fit in the shadow at each eclipse. On one side alone it has 2863 km to spare, far more than the Moon's own diameter.
Because the shadow has so much room, the Moon takes a long time to cross it, and a total lunar eclipse can last over an hour. A total solar eclipse, by contrast, is over in a few minutes. This simple difference in size, with Earth's shadow far larger than the Moon, is also behind the difference in how long the two kinds of eclipse last.
High school+UniversityWhy "red"? A more precise picture
The strength of Rayleigh scattering is thought to be inversely proportional to the fourth power of the light's wavelength. So if the wavelength is halved, the scattering becomes about 16 times stronger. Blue light (wavelength about 450 nm) is scattered far more strongly than red light (about 650 nm). The longer the path through the air, the wider this gap grows.
The light that reaches the Moon in an eclipse is also thought to be affected by absorption from the ozone layer, and by scattering and absorption from clouds and dust. The size of these effects is considered one reason the Moon's colour differs from one eclipse to the next.
ResearchWhat is still not well understood
- It is still considered hard to predict the brightness and colour of the eclipsed Moon precisely in advance. It depends heavily on the state of Earth's atmosphere at the time, such as cloud cover and the amount of volcanic dust.
- Simulation studies that calculate refraction and scattering of light through the atmosphere in detail, to reproduce the colours seen in an eclipse on a computer, are said to still be improving in accuracy.
- Reading the atmosphere of a distant planet from the colour changes of the eclipsed Moon is thought to be a research theme linked to future observing methods for the atmospheres of planets outside the Solar System (exoplanets).
Eclipses have been watched for thousands of years, yet the fine details of the atmosphere that set their colour are still being studied. Being familiar is not the same as being understood.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science: phases of the Moon, solar and lunar eclipses | How a total lunar eclipse works, basics of refraction |
| High school | Basic Earth science: structure of the atmosphere; basic physics: properties of light | Shadow-size calculation, basic idea of scattering |
| High school+ | Earth science: atmospheric optics (advanced) | Rayleigh scattering and wavelength |
| University | Atmospheric science, atmospheric optics | Fourth-power law of wavelength, effects of ozone and clouds |
| Research | Atmospheric science, exoplanet science (unsolved) | Predicting colour in advance, simulation, observing exoplanet atmospheres |
| ― | Everyday observation | Comparison with changing sunset colours |
- Explanatory materials on how solar and lunar eclipses work, from sources such as the National Astronomical Observatory of Japan (国立天文台).
- Lynch, D.K. & Livingston, W., Color and Light in Nature (on atmospheric optics, Rayleigh scattering and the colour of lunar eclipses).
- General descriptions in optics and atmospheric science textbooks of how scattering depends on wavelength (the wavelength dependence of Rayleigh scattering).
- Records and observation data on the Moon's brightness during total lunar eclipses, from astronomical agencies in various countries.
* Figures such as the size of Earth's shadow vary with the positions of the Sun, Earth and Moon. This article gives commonly used approximate values.
* This article is a general-audience science explainer. Figures for the size of Earth's shadow and the wavelength of light vary with conditions, and are given as guides to help you understand the mechanism.