| consists of an icy central part (or nucleus), a head (or coma) and a tail or tails made up of tiny particles of 'dust' together with extremely thin gas. Comets may be enormous (the head of the Great Comet of 1843 was larger than the Sun), but they are very light since the nucleus, the only relatively massive part of a comet, cannot be more than a few miles in diameter. If a comet fell to the earth, it would only cause local damage.
Comets move around the Sun. In almost all cases their paths (or orbits) are elliptical, and except for Halley's Comet, all the really bright comets take thousands or even millions of years to complete one circuit. This means that we cannot predict them. During the last century, several were seen but in our own time they have been
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