Physics Q & A - The red moon?
Cheung Kai-chung (Translation by Yip Ying-kin)    

Fig. 1   Sunlight is refracted by the earth's atmosphere, so part of the sunlight can reach the shadow of the earth.

Why the moon is seen red during total lunar eclipse?

As we all know, total eclipse of the moon happens when the moon orbits into the shadow of the earth. Since the moon is inside the shadow of the earth, how can the moon reflect light and why is the light red in colour? As a matter of fact, this phenomenon is related to refraction. Solar radiation will be refracted when passing through the earth's atmosphere. Therefore part of the sunlight can still reach the shadow of the earth. Besides, the earth atmosphere scatters most of the blue light (see the colour of the sky), so there will be more red light reaches the moon (Fig.1). The red light will be reflected back to earth. That is the reason why you can see a red moon rather than total darkness. It is hoped that you can see the red moon in the next total lunar eclipse!