During a total lunar eclipse, light is bent and filtered as it passes through the atmosphere causing red light to shine on the Moon. To see an eclipse, the Earth must move directly between the Moon and the Sun, causing Earth’s shadow to fall across the lunar surface. At total eclipse, when you would expect the Moon to be totally obscured by Earth’s shadow, the Moon appears red.
As the Earth blocks the path between the Sun and the Moon, sunlight is still able to shine through the halo of gas that surrounds our planet, our atmosphere. However, the light does not make it through entirely unchanged. Light is composed of a spectrum of colours and only the reddish component makes it out the other side.
Answered by Zoe Baily at the National Space Centre