Comet

Comet, astronomical body consisting of a small mass, mostly gas and dust, spread over a large volume, orbiting the sun in a highly elliptical path that may take it as much as 150,000 times as far from the sun as the earth is. The head of a comet is bright, probably composed of ice and frozen gases. As it nears the sun, particles and gases are cast off into a tail that may be as long as 100 million mi (160 million km). The tail always points away from the sun. Some comets are visible from the earth at regular intervals. The most famous of these is Halley's comet, which reappears about every 76 years. Its most recent appearance was in 1986. In the past, comets were superstitiously considered omens of doom. The appearance of Halley's comet in 1066 is recorded on the Bayeux tapestry. De Chéseaux's comet of 1744 showed six separate tails. In 1846 Biela's comet broke in two as it passed the sun. In 1910, shortly before the return of Halley's comet, another comet appeared that was so bright it was visible in daylight. The comet with the shortest periodicity is Encke, which orbits the sun every 3.3 years.

See also: Astronomy.