cmpbsdb@gitpyr.UUCP (Don Barry) (01/12/86)
Anton Winteroak writes: > The answer is that it is unlikely that we will ever know. Even > with all the very accurate data about the last several orbits, the time > of Halley's return this time was uncertain by more than a month. Although we could always use better orbital elements, Halley's was recovered only 2 arc seconds away from its predicted position, and the perihelion date only required a few hours of revision. The orbit is now very accurately known. > The further back we look at the records of Halley's Comet, > the worse the time and position data is. Also, if you note when the > closest approaches were, you'll see that the period has varied considerably. > Most comets keep irregular orbits, since the jetting of material > towards the sun, and with the direction of rotation, causes a small but > signifigant change in momentum. This point is well taken. But in the short run, there are two non-gravitational parameters (which indirectly indicate the spin axis and non-uniform force) are calculated from anomaly from a true orbit, and these only change slightly from orbit to orbit as the axis of rotation slowly precesses. The irregular periods are by and large the result of planetary perturbation, not the comparatively small perturbations induced by jetting. These are also well known and easily calculated to high accuracy by secular perturbation methods or numerical predictor-corrector methods. Of course, there is a limit beyond which we can make no reasonable statements concerning the relative orientation of a comet and perturbing bodies, but with Halley, this limit extends back several thousand years. I'm not fluent in this literature, so I'm not going to say that someone somewhere hasn't found the close-call that put Halley in its present intermediate-class orbit, but I doubt it. At least we can search with confidence back to 2000 B.C. or before for evidence of planetary encounters, both from the comet's point of view, and also as hints of what to look for in civilizations of that era as they witnessed the spectacle we're now enjoying. -- Don Barry (Chemistry Dept) CSnet: cmpbsdb%gitpyr.GTNET@gatech.CSNET Georgia Institute of Technology BITNET: CMPBSDB @ GITVM1 Atlanta, GA 30332 ARPA: cmpbsdb%gitpyr.GTNET%gatech.CSNET@csnet-relay.ARPA UUCP: ...!{akgua,allegra,amd,hplabs,ihnp4,seismo,ut-ngp}!gatech!gitpyr!cmpbsdb