The Orbit of Comets

describe cometary orbits and distinguish them from planetary orbits

In 1705 Edmund Halley predicted, using Newton's new laws of motion, the reappearance of the comet now named after him. It has a period of 76 years last appearing in 1986. It appeared in 1066, an event which is depicted in the Bayeaux Tapestry. If you're lucky enough to be around in 2062 you will see it again.

Two Types of Comet

Comets have been classified as short or long period depending on the orbit time.

Short period	Period is less than 200 years. Those with periods of less than 20 years are believed to originate in the Kuiper belt. Others, e.g. Halley's comet, are thought to originate in the Oort cloud.
Long period	Period greater than 200 years, some have a period of several million. All are believed to originate in the Oort cloud. They have highly elliptical paths.

The orbit of comets is very eccentric (elliptical) unlike that of planets which are almost circular.

Halley's comet is around 35 AU from the Sun at its furthest but just around 0.5 AU at its closest.

If we look down on the solar system from above the earth's North Pole we would see that almost everything orbits the Sun anti-clockwise. This is because the solar system formed from a swirling whirlpool of gas which was spinning this way.

It is thought that the reason why many comets have a retrograde (clockwise) orbit is that they fell into the solar system from the Oort cloud after it had formed.

More evidence from this comes from the fact that the orbit of many comets is not in the ecliptic plane. They could have fallen from the Oort cloud from any direction.