A comet is an irregular body a few kilometers across consisting of rock, ice and frozen gases. Most comets which may be seen by us travel around the Sun in long, oval orbits. When a comet is near the Sun, it develops a long, tenuous tail of gas and dust pointing alway approximately straight away from the Sun.
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Halley's comet is named after Edmond Halley. He thought that comets were bodies of the Solar System in orbits around the Sun. He suggested that a certain comet was a regular visitor, returning every 76 years, and was, in fact, the same one which had been observed since 240 BC, but in particular in the years 1531, 1607, and 1682. In 1682 he calculated its orbit and predicted it would return again in 1758, and sure enough, the comet arrived in March 1759. Halley's comet made a particularly bright appearance in 1910. The last appearance in 1986 was much more modest.
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This simulation shows the orbit of Halley's comet starting on January 1, 2009. Note: The simulation shows a static picture of the comet. The brightness of the comet and the size and direction of the tail varies greatly throughout the orbit.