Nib asked
Why does the Moon turn red during a lunar eclipse?
Earth's shadow is not truly dark. Our atmosphere bends sunlight into it and filters out the blue, so the Moon is lit by the red of every sunrise and sunset on Earth at once.
5 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the lineup
A lunar eclipse happens at full Moon when the Sun, Earth and Moon line up almost exactly, with Earth between the other two. Earth's shadow then falls across the Moon and cuts off the direct sunlight that makes it shine.
In a lunar eclipse Earth sits between the Sun and the Moon and blocks direct sunlight.
2not black
With no direct sunlight, the Moon should go dark, and it would if Earth had no air. In practice a totally eclipsed Moon turns a dim orange or deep red, and stays visible for as long as an hour and a half. Some sunlight must be reaching it by another route.
The shadow is not black: some sunlight still reaches the Moon by a roundabout route.
a common mix-upThe Moon does not turn red because Earth's shadow is red, and it does not give out its own red light. It is lit by sunlight that has passed through our air.
3air bends light
Earth is wrapped in a thin layer of air. Sunlight that grazes the edge of the planet travels through that layer and is refracted, bent inward toward the shadow, in the same way a lens bends light. The bending is small, about a degree, but over the distance to the Moon it is enough to reach the Moon.
Earth's atmosphere refracts sunlight around the planet and into its shadow.
4blue lost, red stays
Air molecules scatter short blue wavelengths much more strongly than long red ones: blue light at 450 nanometres is scattered about six times more than red at 700, which is why the daytime sky is blue. Light that has crossed a long slant path through the atmosphere has lost its blue and arrives reddish, the same effect that makes a sunset red.
Air scatters blue away and lets red through, so the light that bends into the shadow is red.
Why is the light that reaches the Moon red?
Show the answer
The atmosphere scatters out the blue on the long path, leaving the red.
5the whole story
Put it together: Earth blocks the direct sunlight, its atmosphere bends the grazing light into the shadow, the air removes the blue, and the red that remains lights the Moon. You can think of it as all the sunrises and sunsets around the rim of the Earth, seen on the Moon. Volcanic dust or heavy cloud at the rim makes the eclipse darker, so each one has its own colour.
A red eclipsed Moon is lit by the sunrises and sunsets of the whole Earth.
A blood-red Moon is not a warning from the sky, just every sunset on Earth glowing at once.