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Re: Circular orbits, and other stuff



I have another question for those who teach astronomy. The first question
(how is solar spinning angular momentum transferred to the revolving planets
via magnetic forces?) was not answered. I assume it is not a simple topic.

This question has to do with the fact that planetary orbits are often
nearly circular. The same is true for numerous moons and rings. Kepler's
laws would not be violated if eccentricities of planetary orbits were
very large. Why are most orbits nearly circular?

Except for Mercury (e=0.206) and Pluto (e=0.253) the values of e for planets
are smaller than 0.1. For example, 0.007 for Venus, 0.017 for Earth, etc.

How do elipticities change when the spin of our Sun is being transferred to
planets?
Ludwik,
Ok I will bite. The orbits of the planets are nearly circular as a
result of the accretion process that formed them. That is the dust in
elliptical orbits is more likely collide with things since they pass through
the circular orbit material and can collide with other elliptically orbit
dust traveling in different directions as well. By an averaging process the
"preferred material" will be that in circular orbits. The nearly circular
orbiting dust clouds can become concentrated by perturbations from other
clouds in similar orbits and the Poynting-Robertson effect acting on the
dust. Note that magnetic forces are left out of this argument. There are
arguments employing magneto-hydrodynamics, but I am not sure they are
critical to the over all model. Magnetic fields in the solar system are
rather small compared to other forces. In White Dwarf stars, for example,
magnetic breaking is the principle way they lose angular momentum.
One interesting aside is that one of the recently discovered
planetary systems planet is in a very elliptical orbit. The other known ones
are nearly circular.

Gary

P.S. Periapse is when the object is closest to the star, i.e. Perihelion in
the case of the sun, perigee for the earth. Today perturbations of other
bodies and tidal effects are the main causes of the changes in orbital
parameters in the solar system.
Gary Karshner

St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU