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Name: Shelby
Status: student
Grade: 6-8
Location: TX
Country: N/A
Date: 2/1/2005
Question:
Why are the planets' orbits not completely circular?

Replies:
The shape of an orbit is a conic section, in the most general terms. Under certain conditions it can be a parabola or a hyperbola (of course you only get one chance to view the object since these trajectories are not closed. Closed orbits can under special conditions be circular, but far more often are elliptical. The "reasons" have to do with Newton's universal law of gravitation and the conservation of energy and angular momentum. Having said that the actual derivation of the detailed equations require some college level math (vector calculus). If it is any consolation Kepler "knew" the orbits were elliptical by observation a long time before he was able to demonstrate WHY that was the case. The two sites below give the derivation for anyone who wants to follow through on the details.

http://www.math.uab.edu/weinstei/notes/kepler.pdf

http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion

Vince Calder


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