Information based on Encyclopaedia Britannica, 15th edition, 1994:
Comets are small celestial objets of the solar system that travel on extremely elongated elliptical orbits around the sun. They consist of a nucleus and develop a nebulous appearance when they travel through the part of their orbit close to the Sun.
The nucleus consists of frozen matter, most of which is water. Photographs of Halley's Comet taken by the Giotto spacecraft in 1986 show that its nucleus, which is only 8 by 15 km in size, is covered by a crust of dust and appears quite black.
When a comet approaches the Sun the dust on its nucleus heats up, causing the frozen material underneath to turn into gas. This process begins at a distance of about three astronomical units (AU, 1 AU = 150 million km) from the Sun and causes vaporized material - about 80% water, plus in descending order of importance carbon monoxide, carbon dioxide, methane, ammonia and carbon disulfide - to emanate from the comet.
When the comet comes to within 1 AU of the Sun it is surrounded by a gas and dust cloud called "coma" of up to 100,000 km diameter. When it approaches the Sun even more it develops one or two tails. The first, longer tail is produced by the "solar wind," a stream of high energy protons and electrons that sweeps ions from the comet away from the Sun in a straight path. The second, shorter tail is produced by the deflection of dust particles through the solar radiation pressure; it is curved but also points away from the Sun.
Astronomers distinguish short period comets (with a period of 200 years or less) and long period comets (with periods over 200 years). Halley's Comet is one of the best know short period comets and the only one that can easily be seen with the naked eye and returns within the lifespan of humans. It has a period of 76.09 years and was the first whose return was predicted. Edmond Halley (1656 - 1742) showed that a comet observed in 1531, 1607 and 1682 was the same comet and predicted its return for 1758. Its tail is millions of km long. When Halley's Comet passed in 1910 the Earth crossed its tail.
Based on Chinese observations of the 29 appearances of Halley's Comet between 613 BC and 1910 the astronomer J. R. Hind calculated its orbit and showed that it became narrower during the 2149 years of observation. (China Internet Information Center, 2003)
Halley's Comet as it appeared on 4 May 1910 (from the 1911 edition of Encyclopaedia Britannica)
China Internet Information Center (2001)
Formation of the Chinese civilization: Astronomy and Mathematics.
http://china.org.cn/e-gudai/6.htm (accessed 23 November 2003).