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[[Image:Halley Bayeux.jpg|left|150px]]Comets as a class were known to the ancients, and some of the most prominent and regularly appearing comets observed today appear in ancient records. For example, [[China|Chinese]] records mention a recurrent comet beginning in 240 BC. That comet is most likely Comet Halley, whose discoverer, [[Edmond Halley]], was the first to predict a comet's reappearance in advance. Comet Halley also appeared on April 24, 1066, to astounded onlookers in [[England]] prior to the conquest of England by the Normans, as depicted in the Tapestry of Bayeux.<ref name=Bayeux>Crack, Glen Ray. "[http://www.hastings1066.com/bayhi3a.shtml Bayeux Tapestry Highlights Part 3, Image 1]." <http://www.hastings1066.com/>, January 10, 1998. Retrieved July 20, 2008.</ref>

[[Image:Halley Bayeux.jpg|left|150px]]Comets as a class were known to the ancients, and some of the most prominent and regularly appearing comets observed today appear in ancient records. For example, [[China|Chinese]] records mention a recurrent comet beginning in 240 BC. That comet is most likely Comet Halley, whose discoverer, [[Edmond Halley]], was the first to predict a comet's reappearance in advance. Comet Halley also appeared on April 24, 1066, to astounded onlookers in [[England]] prior to the conquest of England by the Normans, as depicted in the Tapestry of Bayeux.<ref name=Bayeux>Crack, Glen Ray. "[http://www.hastings1066.com/bayhi3a.shtml Bayeux Tapestry Highlights Part 3, Image 1]." <http://www.hastings1066.com/>, January 10, 1998. Retrieved July 20, 2008.</ref>

[[Image:Halley1910a.jpg|right|thumb|150px|Halley's Comet on its 1910 appearance. This Yerkes Observatory photograph is highly enlarged; the comet would not have appeared this large or bright to the naked eye.]]In fact, comets have been observed for far longer in history. The ancient [[Greece|Greeks]] called them "evil stars," which in Greek is ''dys'' evil and ''astron'' a star. This is the origin of the modern word ''disaster''. The association of comets with untoward events or prophecies continued as recently as the twentieth century, when Comet Halley made an unusually close rendezvous with [[earth]].<ref name=Newburn>Newburn, Ray. "[http://stardust.jpl.nasa.gov/news/news90.html NASA's Blazing the Trail to Understand Comets]." [[JPL]], [[NASA]], November 21, 2003. Accessed July 20, 2008.</ref>

In fact, comets have been observed for far longer in history. The ancient [[Greece|Greeks]] called them "evil stars," which in Greek is ''dys'' evil and ''astron'' a star. This is the origin of the modern word ''disaster''. The association of comets with untoward events or prophecies continued as recently as the twentieth century, when Comet Halley made an unusually close rendezvous with [[earth]].<ref name=Newburn>Newburn, Ray. "[http://stardust.jpl.nasa.gov/news/news90.html NASA's Blazing the Trail to Understand Comets]." [[JPL]], [[NASA]], November 21, 2003. Accessed July 20, 2008.</ref>

Today, amateur astronomers often discover comets. Recent computerized sky surveys now find most comets as they approach the [[sun]].<ref name=Arnett/>

Today, amateur astronomers often discover comets. Recent computerized sky surveys now find most comets as they approach the [[sun]].<ref name=Arnett/>

== Parts of a comet ==

== Parts of a comet ==

The '''nucleus''' or ''head'' of a comet is a lightweight ball of rock, dust, and ice that resembles a dirty snowball. As a comet approaches the sun at 5 AU or closer, the '''coma''' forms. This is the base of the two ''tails'' of the comet. The ice begins to melt and vaporize, and the solar wind blows the liberated particles away from the sun. The dust forms one tail, and [[ion]]s form another.<ref name=Yeomans/>

[[Image:Halley1910a.jpg|right|thumb|300px|Halley's Comet on its 1910 appearance. This Yerkes Observatory photograph is highly enlarged; the comet would not have appeared this large or bright to the naked eye.]]The '''nucleus''' or ''head'' of a comet is a lightweight ball of rock, dust, and ice that resembles a dirty snowball. As a comet approaches the sun at 5 AU or closer, the '''coma''' forms. This is the base of the two ''tails'' of the comet. The ice begins to melt and vaporize, and the solar wind blows the liberated particles away from the sun. The dust forms one tail, and [[ion]]s form another.<ref name=Yeomans/>

Comet nuclei are currently thought to measure 16 kilometers across or less.<ref name=Yeomans/> A comet nucleus shines with reflected light only. A comet's coma often absorbs ultraviolet radiation and becomes fluorescent. In so doing it can shine more brightly than does the nucleus.<ref name=Hamilton/> A comet's tails can extend for 160 million kilometers<ref name=Yeomans/> and thus appear larger than the constellation of Ursa Major, and occasionally brighter than the [[Milky Way]] itself.

Comet nuclei are currently thought to measure 16 kilometers across or less.<ref name=Yeomans/> A comet nucleus shines with reflected light only. A comet's coma often absorbs ultraviolet radiation and becomes fluorescent. In so doing it can shine more brightly than does the nucleus.<ref name=Hamilton/> A comet's tails can extend for 160 million kilometers<ref name=Yeomans/> and thus appear larger than the constellation of Ursa Major, and occasionally brighter than the [[Milky Way]] itself.