Total lunar eclipse: Once in a red moon

The first total lunar eclipse for four years stunned observers across the world yesterday. Steve Connor explains what caused its dramatic change of colour

If the Moon were made of cheese it would have been a Red Leicester last night for about 50 minutes. That was the time it took to complete a total lunar eclipse, when the Moon's ghostly-white colour turns to a deep orangey-red.

A total lunar eclipse can only occur at a full moon and when the Sun, the Earth and the Moon align in the same plane. As the Moon moves behind the Earth it eventually becomes totally engrossed in our reddish umbral shadow.

The Earth casts two types of cone-shaped shadows. The outer shadow is the penumbra, where the Earth blocks some, but not all of the Sun's rays. When the Moon passes through this shadow, there is a partial eclipse (see main image, to the left and right of the central red Moon). Inside this outer shadow lies the umbra, where the Earth blocks all of the direct rays – but leaves some indirect rays which give a total lunar eclipse its distinctive red coloration.

If you were standing on the Moon at this point in time all you would see of the Earth is a ring of red light – in effect, a series of simultaneous sunsets and sunrises, with only the red elements of the Sun's rays penetrating through the Earth's atmosphere. As a result, the Moon becomes blood-red when viewed from Earth.