Shadow on the Moon

The night's sky often has something amazing to offer - on a clear night stars billions of miles away can be seen and twinkling planets sometimes make an appearance. When staring from Earth it seems impossible that we are moving at approximately 1040 miles per hour, but we are. On Saturday 10th December 2011, a different event at night gave us a glimpse of Earth's position in the Universe.

A lunar eclipse occurs when the Earth moves between the Sun and the Moon, casting a shadow across it. This shadow is rounded and was used by Aristotle to support his theory of a spherical Earth in the 4th century. Unlike a solar eclipse that can only be viewed from certain points on the Earth, a lunar eclipse can be seen from anywhere due to the size of the Earth compared to that of the Moon. The movements of the Earth around the Sun and the Moon around the Earth are clearer than ever when their paths cross.

The 'blood moon', sometimes visible at stages in the eclipse, is so called due to its red colour. The slightly unnerving sight was historically believed to herald a religious coming, but scientifically it shows the presence of the Earth's atmosphere. The coppery-red colour is produced by refracting light as the Sun's rays pass through the atmosphere and into the shadow cast on the Moon. Visible light is within the electromagnetic spectrum and is made up of a rainbow of colours, all of which have different wavelengths. The shorter wavelengths are scattered by the air molecules, pollution and dust found in Earth's atmosphere – so it is the longest red wavelengths that are visible. If the Earth had no atmosphere the Moon would be completely dark during a lunar eclipse.

The event of Saturday 10th December 2011 was the last total lunar eclipse until 2014, the complete blocking of the Sun's rays by the Earth requires a full Moon and the orbits of all three to be exactly or very closely aligned. There will be smaller eclipses between now and then, and unlike solar events a lunar eclipse can be viewed without glasses or eye protection. Tracing a path across the sky and past the Earth, once in a red moon we can see what home could look like from somewhere else in the Universe.