[From Bruce Abbott (951111.1430 EST)]
Bill Leach 951110.22:36 U.S. Eastern Time Zone --
Kepler and Newton
Actually in a sense, Newton no more explained "why" than did Kepler. We
still do not know what Gravity "IS" nor do we know why it functions nor
how. So Newton provided a better functional account than did Kepler plus
he "threw the screws to us" by pointing out that we could not explain how
three such bodies interact at all (ie: We have no formula that predicts
correctly the paths of the sun, earth and moon individually with respect
to each other when accounting for all three)!
I agree, but Newton at least gave a plausible explanation for the orbits in
terms of more fundamental relationships. He derived the motions by assuming
that (a) objects in motion continue in motion along a straight line, unless
acted upon by an external force [inertia], and (b) an external force,
proportional to the masses of the objects and inversely proportional to the
square of their distances apart [gravity], acted constantly on the objects,
the primary relationship (because of its vastly greater mass) being the
"pull" centered on the sun. As you note, Newton did not have an explanation
for this "action at a distance." Even so, his formulation was a vast
improvement over Kepler's "mere" (itself a triumph of imagination)
description of the (to a fair approximation) true planetary motions.
Regards,
Bruce