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Comets of longer period, however, originate far beyond the Kuiper Belt. In 1950, Jan Oort compared the orbits of 21 long-period comets for which accurate orbits were known, and discovered a peculiar tendency. Instead of a random The Motions of Comets 27 scatter of aphelia, there appeared to be a disproportionate concentration at distances between 50,000 and 100,000 AU from the Sun. This tendency was confirmed by a second study in 1953, in which he included a total of 41 comets of very long period.

For the sake of simplicity, the 26 1 The Nature of Comets initial orbit is almost always calculated on the assumption of it being a parabola. Strictly speaking, this assumption is wrong 100% of the time, but it makes for easier math, and the chances are high that the real orbit will be so close to a parabola that the difference will only become apparent after the comet has been tracked across a significant arc of sky. In fact, the difference might be so close as never to be discovered, and the published orbit may never appear as anything other than a parabola.

Vanysek published his results of a somewhat more comprehensive study involving 99 comets observed from 1853 to 1951. 1. Most, though probably not all, of these latter would be new in the sense of Oort and Schmidt. Unless the orbit of a newly discovered comet was quickly found to be an obvious short-period ellipse, chances are it would have a period somewhere between the low hundreds of years and the high hundreds of thousands and so a value of n = 4 might not seem too far off the track unless an obvious departure from this trend became quickly apparent.

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Astronomy (November 2012)

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