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November 21, 2003
A: FROM MENTOR GENAH BURDITT
IN CO
The planets stay where they are because all of the planets
and the sun remain in an equilibrium between their respective
gravities. Pluto does come closer to the sun than Neptune
every few hundred years but it's not because they switch places,
it is because Pluto's orbit is highly elliptical (oval) instead
of rather circular like Neptune's. Therefore, sometimes it
is just a little bit closer to the sun.
A great program that you try for free on the internet, that
allows you study TONS of stuff about planetary motion is called
Starry Night Pro. Just type this into GOOGLE and see what
you can find. Another fun site you can find by typing the
following into google: BBC space science games.
Good luck!
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November 19, 2003
A: FROM MENTOR MOLLY WILLIAMS
IN MI
Hi Crystal,
You have two questions here. First, the planets stay in orbit
because the gravitational pull of the Sun keeps them from
traveling in a straight line. Their path gets bent into a
curve. Similarly, the Moon stays in its orbit around the Earth,
because of the pull of the Earth. Most of the planets in our
solar system are in orbits that are nearly circular. We suspect
that they coalesced out of a cloud of spinning dust (imagine
a big, spinning cookie), so their orbits just preserved the
shape of the original circular mass. However, sometimes orbits
can be elliptical. Comets, for example, have very "eccentric"
orbits, meaning that their distance from the sun varies greatly
throughout their orbital year. Some comet orbits get as close
to the sun as the inner planets (Earth or Mars) and then get
so far away that they are outside Pluto. Pluto has a much
more eccentric orbit than the other planets. Its orbit may
have been disturbed by another large object that passed by,
or it might have been captured rather than forming at the
same time as the other planets. So, the answer to the second
question is that sometimes Pluto can be closer to the Sun
than Neptune, even though on average Neptune is closer to
the Sun. But Mars and Earth are in nearly circular orbits;
they keep almost the same distance from the Sun at all times,
so they can't switch places.
NASA has a really good website about the Solar System: http://sse.jpl.nasa.gov/planets/index.cfm
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