Planetary Eccentricity



name         Jennifer
status       student
grade        9-12
location     IA

Question -   Why are the planet's orbits an elliptical shape instead of a
circle?  What causes the planets to be farther from the sun at certain
times during its orbit?
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This is because it is very hard to get a circle. It is almost like getting a
straight line, almost an impossibility.

The reason why planets in a multi-planet solar system have elliptical
orbits, is that there are too many forces pulling what could be a circular
orbit out of shape. There are too many gravitational forces from other
planets which render a circular orbit virtually impossible.

Your second question is basically answered by the fact that, concerning a
long-life orbit (like Earth's), the Sun is always at one focus of the
ellipse, hence perihelion (the closest point to the Sun) is directly
opposite aphelion (the furthest point from the Sun).

Howard Barnes.
Astronomer.
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  The general solutions to the laws of gravitational mechanics predict
that ellipses are the trajectories of planets about the sun, or any two
bodies moving about one another as the result of a 1/R^2 gravitational
force. Kepler first observed the behavior and Newton derived the mechanical
laws that dictate this.
   I don't know of a short and simple derivation of why it turns out this
way, but there are a number of websites, for example below, that go into
more detail than can be done on a forum such as NEWTON

http://library.thinkquest.org/04apr/00533/Text-Only/kepler's_laws.htm

http://curious.astro.cornell.edu/question.php?number=467

http://www.windows.ucar.edu/tour/link=/the_universe/uts/kepler1.html&edu=high

http://en.wikipedia.org/wiki/Orbit

Vince Calder
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