On 11 Oct 2005 20:27:05 -0700, "tt40" wrote:

In everything I've read about planets and elliptical orbits, I can't
ever recall any author (Feynman, Newton, 'Ask an Astronomer' etc.),
explaining exactly 'why' the orbit is elliptical. Oh sure there's been
lots of mathematics to explain the orbit and how it works, but most of
the explanations don't provide a definitive statement as to why it IS
elliptical.


Until someone can do the 'why', I'm going to assume that either:

1. it's because the Sun is in a rotation of it's own and all the
planets have to scoot to 'catch' up with it. But since the Sun is
moving too, the now-receding planet's momentum carries it further away
than it expected, so it maps out a stretched circle, longer than it
thought was necessary. So it's stretching the distance on the long-arm
of the orbit and has to scoot back in again, chasing that moving Sun.

or

2. the minute gravitational tug of the planet pulls the Sun closer (out
of its 'fixed' position relative to the planet) which increases the
mutual gravitational attraction so that they are each attracted that
little bit stronger. But since the Plant is still in orbit, it follows
the short arm of the ellipse that little bit faster or a little more
energised. It then maps out the long-arm of the ellipse and
gravitational attraction recedes just a little bit, allowing it to
'stretch that circle'.

The correct answer is found in the mathematics. Any attempt to explain
it in English is a poor approximation at best (your first suggestion is
entirely incorrect; you are sort of on the right track with the second).

In a two body system, with the bodies acting under the force of gravity,
an elliptical path is simply what happens. The math that describes that
is really the answer.

http://en.wikipedia.org/wiki/Kepler%27s_laws
http://astron.berkeley.edu/~converse/Lagrange/Kepler'sFirstLaw.htm

Maybe Brian Tung will chip in. He has a talent for putting the math into
English.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com