An Etymological Dictionary of Astronomy and Astrophysics

The → path followed by a → comet in the → solar system around the → Sun.

Most cometary orbits appear to be → elliptical, or in some cases → parabolic. The orbits of → short-period comets are elliptical, carrying them out to a region lying from → Jupiter to beyond the orbit of → Neptune. Those of → long-period comets are very elliptical. The orbits may be strongly influenced if they pass near the Jovian planets, particularly Jupiter itself. The cometary orbits are also influenced to some degree by gases shooting out of comets, so their orbits are primarily but not completely determined by gravity.
Newton (1644-1727) was the first to compute a cometary orbit. He found that the comet of 1680 was following a parabolic orbit around the Sun. Edmond Halley (1656-1742), following the methods of Newton, computed the → orbital elements of 24 comets. He realized that the comets of 1531, 1607 and 1682 had very similar elements and postulated that they were in fact the same object, orbiting an elongated ellipse. He predicted the next return to occur in 1758 or early 1759. The return of what is now called Halley’s comet was observed after his death, This first observation of a “predicted” comet is manifestly one of the major successes of → celestial mechanics.

See also: → cometary; → orbit.