Total lunar eclipse, 3 March 2007.
© Jamie Cooper / Science & Society Picture Library
The red colouring arises because of the scattering of sunlight in the Earth's atmosphere. Sunlight reaching the Moon must pass through a long and dense layer of the Earth's atmosphere, where it is scattered by dust particles. Shorter wavelengths are more likely to be scattered by the small particles, and so by the time the light has passed through the atmosphere, the longer wavelengths dominate. This resulting light we perceive as red. This image was taken towards the end of totality.