The return of Comet Halley
Halley demystified comets by showing that at least one of them travels in an elongated orbit around the Sun. He found that the orbit of the comet of 1682 was similar to those of comets observed in 1607 (by Johannes Kepler, 1571-1630) and in 1531 (by Petrus Apianus, 1495-1552). All three comets moved around the Sun in retrograde orbits with a similar orientation. Halley also knew that the Great Comet of 1456 had traveled in the retrograde direction, and he concluded that all four comets were returns of the same comet in a closed elliptical orbit around the Sun with a period of about 76 years. Halley confidently predicted its return in 1758, noting that he would not live to see it. After the comet was re-discovered, on Christmas night of the predicted year, Halley’s achievement was acknowledged, albeit posthumously, by naming it Comet Halley.
After its 1910 apparition, Comet Halley moved away from the Sun into the outer darkness, arriving in 1948 at the remotest part of its orbit at 35 AU, or at 35 times the Earth’s distance from the Sun. The comet then turned the direction of its course, and began falling back toward the heart of the solar system with ever-increasing speed. It reached perihelion or its closest distance from the Sun, on 9 February 1986. Comet Halley and the Earth were then on opposite sides of the Sun, so this was among the least favorable apparitions for observing the comet with the unaided eye. Nevertheless, it still became one of the most thoroughly studied apparitions in the history of comet research, including visits by six spacecraft.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University