During a lunar eclipse, the Moon appears red because it is directly illuminated by sunlight that has passed through Earth's atmosphere, indicating that the Earth's atmosphere scatters shorter wavelengths of light more than longer wavelengths.

During a lunar eclipse the Earth sits between the Sun and the Moon, so the Moon receives only sunlight that has gone through Earth’s atmosphere. That atmosphere removes most of the blue and green light by scattering it out of the beam, leaving mainly the red and orange wavelengths. The remaining light then falls on the Moon, giving it a reddish hue. For example, if you shine a flashlight through a glass of water, the blue part of the beam is scattered and the beam that reaches the other side is mostly red. This scattering process is the same that makes daytime sunsets red.