Halley's Comet is the best-known of the short-period comets, and is visible from Earth every 75 to 76 years. Halley is the only short-period comet that is clearly visible to the naked eye, and thus, the only naked-eye comet that might appear twice in a human lifetime. Other naked-eye comets may be brighter and more spectacular, but will appear only once in thousands of years. Halley's returns to the inner solar system have been observed by astronomers since at least 240 BC, and recorded by Chinese, Babylonian, and mediaeval European chroniclers, but were not recognised as reappearances of the same object. The comet's periodicity was first determined in 1705 by English astronomer Edmond Halley, after whom it is now named. It last appeared in the inner Solar System in 1986 and will next appear in mid-2061. During its 1986 apparition, Halley's Comet became the first to be observed in detail by spacecraft, providing the first observational data on the structure of the comet nucleus and the mechanism of coma and tail formation. These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple's "dirty snowball" model, which correctly surmised that Halley would be composed of a mixture of volatile ices, such as water, carbon dioxide and ammonia, and dust. However, the missions also provided data which substantially reformed and reconfigured these ideas; for instance it is now understood that Halley's surface is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.
Read the rest of this article: <http://en.wikipedia.org/wiki/Halley%27s_Comet> _______________________________ Today's selected anniversaries: 1744: War of the Austrian Succession: British ships began attacking the Spanish rear of a Franco-Spanish combined fleet in the Mediterranean Sea off the coast near Toulon, France. <http://en.wikipedia.org/wiki/Battle_of_Toulon_%281744%29> 1819: Under the terms of the Adams–Onís Treaty, Spain sold Florida and other North American territory to the United States for about US$5 million. <http://en.wikipedia.org/wiki/Adams%E2%80%93On%C3%ADs_Treaty> 1943: Members of the White Rose, a nonviolent resistance movement in Nazi Germany that became known for a leaflet campaign that called for active opposition to Adolf Hitler's regime, were found guilty of treason and guillotined. <http://en.wikipedia.org/wiki/White_Rose> 1959: Lee Petty won the first Daytona 500 NASCAR auto race at the Daytona International Speedway in Daytona Beach, Florida, US. <http://en.wikipedia.org/wiki/Daytona_500> 2002: Jonas Savimbi, leader of the Angolan anti-Communist rebel and political party UNITA, was killed in a battle with Angolan government troops. <http://en.wikipedia.org/wiki/Jonas_Savimbi> _____________________________ Wiktionary's word of the day: grommet (n): 1. A reinforced eyelet, or a small metal or plastic ring used to reinforce an eyelet. 2. (sports) A young or inexperienced surfer, skateboarder or snowboarder <http://en.wiktionary.org/wiki/grommet> ___________________________ Wikiquote quote of the day: The Citizens of the United States of America have a right to applaud themselves for giving to Mankind examples of an enlarged and liberal policy: a policy worthy of imitation. All possess alike liberty of conscience and immunities of citizenship. It is now no more that toleration is spoken of, as if it was by the indulgence of one class of people that another enjoyed the exercise of their inherent natural rights. For happily the Government of the United States, which gives to bigotry no sanction, to persecution no assistance, requires only that they who live under its protection should demean themselves as good citizens in giving it on all occasions their effectual support. --George Washington <http://en.wikiquote.org/wiki/George_Washington> _______________________________________________ Wikipedia Daily Article mailing list. To unsubscribe, visit: https://lists.wikimedia.org/mailman/listinfo/daily-article-l Questions or comments? Contact [email protected]
