Science Puzzle
Why Does the Moon Turn Red in an Eclipse?
In a total eclipse of the Moon, the Earth passes directly between the Sun and the Moon, and the Moon moves into Earth's shadow. You might expect it to vanish. Instead it glows a deep coppery red, which is why people call it a blood moon.
Why does the Moon turn red instead of disappearing?
The Answer
It's lit by sunlight that has passed through Earth's atmosphere. The air bends some light round into the shadow, and on its long journey through the air most of the blue is scattered away, so only reddish light reaches the Moon.
Earth blocks the direct sunlight, but it has a thin shell of air round it. Sunlight skimming through that air is bent slightly, the way a lens bends light, and some of it is steered into the shadow. That light has travelled a very long way through the atmosphere, and air scatters blue light much more than red. It's the same reason sunsets are red. By the time the light reaches the Moon, it's mostly red and orange.
If you stood on the Moon during the eclipse, you'd see the Earth as a black disc ringed with a thin, glowing red line: every sunrise and sunset on Earth, all at once. That ring is what lights the Moon.
No two eclipses look quite the same, because the colour depends on how clean Earth's air is. After big volcanic eruptions fill the high atmosphere with dust, the eclipsed Moon can turn so dark it's hard to see: that happened in December 1992, the year after Mount Pinatubo erupted.
The principle: Refraction and scattering together. Earth's atmosphere bends a little sunlight into its shadow and scatters away the blue on the way, so the eclipsed Moon is lit by the red light of every sunrise and sunset on Earth.