Comet (Latin, stella cometa, “hairy star”), celestial body of nebulous appearance, revolving round the Sun. A comet is characterized by a long, luminous tail, but only in that part of the comet’s orbit in which it passes closest to the Sun.

Appearances of large comets were regarded as atmospheric phenomena until 1577, when the Danish astronomer Tycho Brahe proved that they were celestial bodies. In the 17th century the English scientist Isaac Newton demonstrated that the movements of comets are subject to the same laws that control the planets in their orbits. By comparing the orbital elements of a number of earlier comets, Edmond Halley showed the comet of 1682 to be identical with the two that had appeared in 1607 and 1531, and he successfully predicted the return of the comet, which reappeared in 1758. The earlier appearances of Halley’s comet have now been identified from records dating from as early as 240 BC. Halley’s comet passed round the Sun most recently early in 1986. As it once more headed outward, it was visited in March of that year by two Soviet-constructed probes, Vega 1 and 2, and by another instrumented package called Giotto, launched by the European Space Agency. Two Japanese craft observed it from a great distance as it passed.