When Chiron was Perturbed

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 

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