'Comets', c 1860.
© Science Museum / Science & Society Picture Library
One of a set of teaching cards published by James Reynolds & Sons, London. The illustration, by John Emslie, shows comets from history; the comets of 1680, and 1811, Donati's Comet of 1858, Halley's Comet of 1835, and the multi-tailed comet of 1741. 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 comet's surface to evaporate, forming the distinctive tail of material, often millions of kilometres long, which always points away from the Sun. Knowledge of comets dates back to antiquity, and their appearance was often regarded as heralding momentous events.