Grades 3–5 · Earth & Space
Why can the Moon look red during a lunar eclipse?
The Moon shines by reflecting sunlight. During a lunar eclipse, the Moon moves through Earth’s shadow. Earth is between the Sun and Moon, blocking some or all of the direct sunlight that would reach the Moon.
In the diagramSome sunlight reaches Earth’s shadowSunEarth + airMoonblue light scattersreddened lightSchematic eclipse geometry; ordinary Moon phases are not Earth’s shadow.
During a total lunar eclipse, some sunlight still reaches the Moon after passing through the edge of Earth’s atmosphere. The atmosphere bends this light toward the shadow. It also scatters away much of the bluer light.
The remaining light is richer in red and orange. When the Moon reflects it toward us, the Moon can look rusty red rather than disappearing. Clouds and particles in Earth’s air affect how bright or dark it looks.
A lunar eclipse requires a close alignment near full Moon. Most full Moons miss Earth’s shadow because the Moon’s orbit is tilted. The ordinary monthly phases are different: they come from our changing view of the Moon’s sunlit half, not Earth’s shadow.
About this explanation
Starting knowledge: Know that the Moon reflects sunlight and that Earth has an atmosphere.
Sources
- NASA — Eclipses and the Moon