A photo of the May 2003 lunar eclipse by Loyd Overcash of Houston, Tex.

When the Moon’s orbit takes it into the Earth’s shadow, it loses its full Moon brilliance and becomes dark. This is because the Moon has no light of its own – you can only see it when the Sun shines on it. So if the Earth blocks its light source, it appears to go dark. But it’s not true to say that absolutely no Sunlight reaches the Moon at this time. Some rays from the Sun reach the Moon by being bent around the Earth by our planet’s atmosphere. Small as this amount of refracted light may be, it does stop the Moon from disappearing totally from our skies during the eclipse. It’s also responsible for shading the Moon with colour.

The Sun’s light includes all the colours of the spectrum, but only red light can penetrate through the gas and dust in the Earth’s atmosphere to reach the Moon. If the atmosphere is relatively clean and only a small amount of dust is present, then more than just the red end of the spectrum can filter through, making the Moon’s dark surface appear washed in a feeble grey or yellow hue. Even more dust gives it a tobacco colour. The colour range runs right through to blood red, when a large amount of dust is mixed in with the natural gases of the Earth’s atmosphere.

Volcanic eruptions in the twelve months prior to the eclipse are thought to be the major contributor: they eject dust high into the air, which prevailing winds then carrying around the globe. Though there are many references in literature to blood red Moons and their supposed foretelling of impending calamity, in reality it is only a report card on the condition of the Earth’s atmosphere, and a spectacularly dramatic sight.