The Orbits of Comets
Although the occurrence of many comets had been recorded, it was not until 1577 that the Danish astronomer Tycho Brahe suggested that they traveled in elongated rather than circular orbits. A century later Giovanni Borelli concluded that the orbits were parabolic and that comets passed through the solar system but once, never to return. In 1705, however, Edmond Halley concluded that the comet observed in 1682 was the same one that had been described in 1531 and 1607, and he predicted that it would return again in late 1758 or early 1759. The comet was sighted on Christmas Day in 1758, and it returned again in 1835, 1910, and 1986 (see Halley's comet). While some comets appear to have parabolic orbits (see parabola), others return to the inner solar system in highly elongated orbits with periods ranging from a hundred to thousands of years. Still others return at shorter intervals of less than 10 years and reach aphelion (the orbital point farthest from the sun) near the planet Jupiter; these have been captured into their smaller orbits by Jupiter's gravitational attraction.
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