Blood Moon: Why The Moon Turns Red During a Total Lunar Eclipse

The Moon turns red during a total lunar eclipse. But if the Moon is completely in Earth’s shadow, then how does it turn red?

The atmosphere of the Earth acts like a giant lens and refracts some of the sunlight into the surface of the Moon.

When a total lunar eclipse occurs, the Moon is completely in Earth’s shadow, and does not get sunlight directly from the Sun. The Moon is not completely dark during a total lunar eclipse because some of the sunlight passes through the atmosphere of the Earth and gets refracted into the surface of the moon.

Sunlight is composed of different colors. These colors are visible through a prism, or in rainbows, for example. When sunlight enters Earth’s atmosphere, it is affected by a phenomenon called the Rayleigh scattering.  Colors towards the blue spectrum are scattered more than colors towards the red end of the spectrum. This is why the light that reaches the surface of the Moon is dominated by reddish part of sunlight. The blue components are ‘left behind’ on the atmosphere of the Earth, and causes the blue color of our sky.

The same phenomenon also causes the red-orange color when the Sun is low in the sky, specifically at sunrise and sunset.