Why is a blood moon called so?

Sky gazers in India and in other parts of the world will be able to witness a blood moon on September 7 during a total lunar eclipse. The moon will take on a dark red-copper hue. This is the result of a physical effect called Rayleigh scattering. 

During a total lunar eclipse, the earth comes between the sun and the moon, blocking direct sunlight from striking the lunar surface. However, not all sunlight is blocked. Only the bluer light is filtered out; the redder light is scattered by the earth’s atmosphere, giving the moon its striking colour.

This phenomenon is called Rayleigh scattering. The British Nobel laureate John William Strutt (Lord Rayleigh) explained the phenomenon in the 19th century. When light interacts with particles smaller than its wavelength, the intensity of the scattered light is inversely proportional to its wavelength. This is why earthsky appears blue: it has the shortest wavelength in visible light.

During a blood moon, however, the bluer light is absorbed by the earth’s atmosphere while the redder light is refracted towards the moon. The precise hue depends on dust and smoke levels in the atmosphere.