Time exposure of star trails over the dome of the 4.2 metre (165-inch) William Herschel Telescope at the Roque de los Muchachos Observatory on La Palma, in the Canary Islands. The camera is pointing at the pole star, Polaris, so that the stars trace out circular arcs as a result of the Earth's motion. The different colours of the star trails are due to the different intrinsic colours of the stars themselves. Commissioned in 1987 & operated by the Royal Greenwich Observatory, the Herschel is the third largest single mirror telescope in the world. The observatory is located at an altitude of 2400 metres (8000 feet).