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D.K. Yeomans (JPL/Caltech)
Three centuries ago, Edmond Halley published a physically unimpressive treatise of only two dozen pages that was to change the astronomy of comets forever. Using an orbit determination technique developed by his colleague, Isaac Newton, Halley undertook what he termed “a prodigious deal of calculation” to determine the parabolic orbital elements for two dozen comets observed from 1337 through 1686. Not only did Halley note the orbital similarities of the comets observed in 1531, 1607, and 1682 and predict the 1758 return of the comet that would bear his name, but he made several other insightful comments as well. Halley noted that the cometary orbits had no preferred inclination or perihelion distance, nor were any found to be on hyperbolic trajectories. He concluded that there must be a far greater number of them moving in a region remote from the sun and they probably move in very eccentric orbits and make their returns after long periods of time. He called out that some comets make close Earth approaches and these might be used to determine the length of the astronomical unit if parallax measurements could be undertaken. In a final prescient note in his little treatise, Halley noted the distinct possibility that the Earth could be struck by a comet. In an earlier 1694 treatise, Halley noted that the Caspian Sea and other great lakes of the world may result from such cometary collisions.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.