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Old 10-08-2019, 07:43 AM
DPRK is offline
Join Date: May 2016
Posts: 3,976
Originally Posted by The Stafford Cripps View Post
But if the time is important then that is effectively a measure of apparent speed, no? I'm trying to intuitively (ie I didn't take trigonometry past the age of 17) understand how you would tell the difference between a highly elliptical comet and an asteroid with a more circular orbit.
If you assume the orbit is an ellipse or conic section centered on the sun- indeed, Kepler's name is associated with this- then once you fit it to observations the relevant parameter is the eccentricity, which is a dimensionless number with no units. Circular orbits will have eccentricity close to 0, while the comet will have an eccentricity close to 1.

If you look at the last couple of pages in Frankenstein Monster's link, you can see for instance how he rules out a parabolic orbit for Uranus, demonstrating it must be a genuine planet.

ETA Kepler's laws also imply that the speed changes depending on the distance from the sun, so a comet moves faster through space as it approaches the sun

Last edited by DPRK; 10-08-2019 at 07:47 AM.