The best known comet of all is Halley, which has returned with a 74-79
year period since 240 B.C. The following image
shows a famous view of the
full tail of Halley recorded from the Mount Wilson Observatory.
Comet Halley from Mount Wilson |
Edmund Halley and His Comet
The English astronomer
Edmund Halley
was a good friend of Isaac Newton. In 1705 he used Newton's new theory of
gravitation to determine the orbits of comets from their recorded positions in
the sky as a function of time. He found that the bright comets of 1531, 1607,
and 1682 had almost the same orbits, and when he accounted for the
gravitational perturbation on the cometary
orbits from Jupiter and Saturn, he concluded
that these were different appearances of the same comet. He
then used his gravitational calculations to predict
the return of this comet in 1758.
A Posthumous Christmas Present
Halley did not live to see his prediction tested
because he died in 1742. But on
Christmas night, 1758, the comet destined everafter to bear Halley's name
reappeared in a
spectacular vindication of his bold conjecture
and of Newton's gravitational
theory. Tracing back in the historical records for recordings of bright comets
and their positions in the sky, it was concluded that Halley
had been observed periodically as far back as 240 B.C. The most recent
return was in
1986, and the predicted next appearance of Halley in the inner Solar System
will be in 2061.
|
The Head of Halley |
|
The Orbit of Halley's Comet
The following figure shows the orbit of Halley's Comet and its predicted
location in 2024 relative
to the orbits of the planets.
Halley's Comet in 2024 |
Blue is above the plane of the
ecliptic and green is below. Almost the entire Halley orbit is below the plane
of the ecliptic. Further, Halley revolves around its orbit in retrograde
motion (the opposite sense from planet revolution). In the preceding view the
planets revolve counter-clockwise and Halley revolves clockwise.
The following
image shows the same thing, but from a top view.
Halley's Comet in 2024---Top View |
Notice that Halley's orbit
extends essentially to the distance of Pluto, but when Halley is at its
greatest distance from the Sun (
aphelion)
it is below the plane of the ecliptic (green color) while that portion of
Pluto's orbit is above the plane of the ecliptic (blue color).
The following image illustrates this more clearly. It is a view
of the orbit of
Halley and its 1996 position from the vantage of the ecliptic plane:
Solar System View from the Ecliptic Plane |
This view illustrates clearly four important features of our Solar System:
- How close to a plane the orbits of all planets
but Pluto lie.
- The large tilt of Pluto's orbit out of the ecliptic plane.
- How elliptical Comet Halley's orbit is.
- How Halley's orbit lies well below the plane of the ecliptic
when it is in the outer Solar System.
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