The comet of 1680.
© Science Museum / Science & Society Picture Library
Painting. Comets are made up of a mixture of frozen water and gases and dust, and are relics left over from the formation of the Solar System. Most have highly elliptical orbits which periodically bring them in close to the Sun. When this occurs solar radiation causes material on the surface of a comet's nucleus to evaporate, forming the distinctive tail of material, often millions of kilometres long, which always points away from the Sun. The tail of the comet of 1680 was estimated to be 30,000,000 miles in length. Knowledge of comets dates back to antiquity, and their appearance was often regarded as heralding momentous events. By the late 17th century this was beginning to give way to rational scientific enquiry regarding their origin and behaviour.