Two different reasons
There are two common situations in which the Moon looks red, and they share the same basic physics:
- During a total lunar eclipse, the whole Moon sits inside Earth’s shadow and can glow copper, orange or deep red for over an hour.
- When the Moon is low in the sky, near moonrise or moonset, it often looks orange or reddish for a short time, much like the Sun at sunset.
In both cases the color comes from Earth’s atmosphere, not from any change on the Moon itself. The lunar surface is a dull gray; it simply reflects whatever light reaches it.
Rayleigh scattering: why red light survives
Sunlight contains all colors. When it passes through air, the tiny molecules of nitrogen and oxygen scatter short wavelengths, such as blue and violet, much more strongly than long wavelengths, such as orange and red. This effect is called Rayleigh scattering. It is why the daytime sky is blue: scattered blue light reaches you from every direction.
The longer the path light takes through the air, the more blue is removed. What is left is mostly red and orange. That is why sunsets are red, and it is also why a rising or setting Moon can look orange: its light crosses far more atmosphere near the horizon than when it is overhead.
Refraction: how light gets into Earth’s shadow
During a lunar eclipse, Earth sits directly between the Sun and the Moon. You might expect the Moon to vanish completely, but it rarely does. Earth’s atmosphere acts like a weak lens. Sunlight that grazes the edge of the planet is bent, or refracted, inward and into the central part of Earth’s shadow, called the umbra.
That light has just passed edgewise through a long stretch of atmosphere, so most of its blue has already been scattered away. The light that reaches the Moon is the reddened glow of every sunrise and sunset happening around Earth at that moment. An astronaut standing on the Moon during totality would see Earth as a dark disk ringed by a thin, bright red-orange band.
Why some eclipses are darker than others
No two total lunar eclipses look quite the same. The brightness and color depend on:
- Dust and volcanic aerosols. After a major volcanic eruption pushes particles high into the stratosphere, eclipses can be very dark, with the Moon almost disappearing.
- Clouds along the rim of Earth where the light passes, which block part of it.
- How deep the Moon goes into the shadow. The center of the umbra is darker than its edges, so a Moon that passes through the middle tends to look darker and redder than one that skims the edge.
Astronomers describe the result with the Danjon scale, which runs from L0 (very dark, the Moon nearly invisible) to L4 (bright copper or orange, often with a bluish or bright rim). Observers often estimate the Danjon value by eye during totality.
The blue band at the edge
Careful observers sometimes notice a faint turquoise or blue fringe at the edge of the shadow during the partial phases. This comes from light that passed through the ozone layer high in the atmosphere. Ozone absorbs some red light, leaving a hint of blue. It is subtle, and binoculars help.
Smoke, haze and a red Moon on ordinary nights
Large wildfires, dust storms and heavy pollution can add enough particles to the air to tint the Moon orange or red even when it is high up. This is the same filtering at work, just with larger particles doing part of the job. It is not related to an eclipse and can happen at any phase.
Seeing it for yourself
A red eclipsed Moon is completely safe to watch with the naked eye. Check the eclipse calendar for upcoming dates, and read how to view a lunar eclipse for timing and equipment tips. For a red Moon near the horizon, look up local times on the moonrise and moonset page, and around full Moon on the moon phases calendar. The popular name for an eclipsed red Moon is covered in what is a blood moon.