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by Rique Pottenger
When ACS changed over to the latest
JPL ephemerides , and I re-ran the numerical integration program for
Chiron to get more accurate positions. I was quite surprised to see the
difference between the old and new positions go from a few minutes of arc in
1000AD to two signs (!) in 500AD.
Upon investigation, I discovered
that Chiron made a very close approach to Saturn in the second decade of the
eighth century, and a very close approach to Uranus in the ninth decade of the
sixth century. The combined effect of these gravitational yanks changed its
orbit considerably.
Chiron is a planetoid orbiting
between Saturn and Uranus. It is probably either a captured comet, or a former
satellite of one or the other. Its orbit is sufficiently elliptical that it
comes inside the orbit of Saturn when it is closest to the Sun, and almost
reaches the orbit of Uranus at its most distant point.
A numerical integration position
program starts from a known position and velocity and works forwards and/or
backwards. Each step looks at the gravitational influences on the planet being
calculated by the other planets (including the Sun, of course). In a simple two
body system, like a sun and a planet, the planet traces an ellipse around the
sun. But when other bodies are present, the pull of their gravity changes the
course of the planet. Astronomers call these gravitational pulls perturbations.
So Chiron was strongly perturbed.
These pictures are generated by
the ACS Astrological Mandalas program,
which calculates positions for a given pair of planets at intervals, and draws
lines between them. The Solar System is viewed from "above" (celestial
north), so the planets move counterclockwise around the Sun. This plot starts in
699 AD. Chiron starts outside Saturn's orbit, crosses inside in 713, the two
bodies are conjunct and within one AU (Astronomical Unit, the distance from the
Earth to the Sun) of each other in 718, and Chiron crosses back outside Saturn's
orbit in 720-721.
Here is a plot of Chiron and
Uranus. Uranus is the outer planet throughout. As you can see, the two bodies
came very close (again within one AU) in 585-586AD.
The net effect of these close
encounters is that the position of Chiron is somewhat uncertain between 586 and
718 AD, and very uncertain before 585 AD |
