Comet Facts: From Black Death to Deep Impact

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• Many scientists believe that a comet or asteroid struck Earth about 65 million years ago near Mexico's Yucatán Peninsula. The cataclysmic event may have caused widespread extinction of the dinosaurs and three-fourths of Earth's living organisms.

• Comets visible to the naked eye appear in the sky about once every five years but don't always make a dramatic scene. Distinctive, bright comets with long tails appear only every 10 to 12 years or so.

• We see comets because their dust reflects sunlight from the void of space and because some comet gases absorb ultraviolet light and glow, much like fluorescent lightbulbs.

• Ancient peoples of many cultures found religious and cultural signposts in the regular movements of celestial bodies such as the sun, moon, and planets. The seemingly random and surprising appearance of comets was often seen as a harbinger of momentous, sometimes ominous, events, such as death, war, or disaster.

Such portents appear in accounts of many legendary and historic events, including the assassination of Roman Emperor Julius Caesar, the onset of the Black Death plague during the Middle Ages, and the arrival of Francisco Pizarro, the Spanish conqueror of the Inca.

• Ancient Chinese astronomers carefully tracked and recorded comets as far back as the Han dynasty in the third century B.C. These records were important in allowing later astronomers to determine the true nature of the celestial bodies.

• Comets were considered random occurrences until the late 1600s and early 1700s. The famous Halley's comet was the first to be identified as a recurring event.

Edmund Halley studied the historical orbits of comets that appeared in 1531, 1607, and 1682 and concluded that they were the same body. He correctly predicted the 1758 return of the comet that now bears his name. But Halley died in 1742, before he could be proven correct.

• The most famous of comets, Halley's comet orbits the Earth every 76 years and next returns in 2061.

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