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If one could look down on the solar system
from far above the North Pole of Earth, the planets would appear to move
around the Sun in a counterclockwise direction. All of the planets except
Venus and Uranus rotate on their axes in this same direction. The entire
system is remarkably flat�only Mercury and Pluto have obviously inclined
orbits. Pluto�s orbit is so elliptical that it is sometimes closer than
Neptune to the Sun.
The satellite systems mimic the behavior of their parent planets and move
in a counterclockwise direction, but many exceptions are found. Jupiter,
Saturn, and Neptune each have at least one satellite that moves around the
planet in a retrograde orbit (clockwise instead of counterclockwise), and
several satellite orbits are highly elliptical. Jupiter, moreover, has
trapped two clusters of asteroids (the so-called Trojan asteroids) leading
and following the planet by 60� in its orbit around the Sun. (Some
satellites of Saturn have done the same with smaller bodies.) The comets
exhibit a roughly spherical distribution of orbits around the Sun.
Within this maze of motions, some remarkable patterns exist: Mercury
rotates on its axis three times for every two revolutions about the Sun;
no asteroids exist with periods (intervals of time needed to complete one
revolution) 1/2, 1/3, ..., 1/n (where n is an integer) the period of
Jupiter; the three inner Galilean satellites of Jupiter have periods in
the ratio 4:2:1. These and other examples demonstrate the subtle balance
of forces that is established in a gravitational system composed of many
bodies.
Microsoft� Encarta� Reference Library 2003. � 1993-2002 Microsoft
Corporation. All rights reserved.
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