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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Delta-T
From: Frank Reed CT
Date: 2004 Dec 4, 00:29 EST
From: Frank Reed CT
Date: 2004 Dec 4, 00:29 EST
George H wrote:
"Not to us,
nor to many generations ahead. But the time-scales involved in significant
changes to planetary orbits, such as that of the Earth, are not long in
comparison with the time-scales for biological evolutionary changes here on
Earth. Such orbital changes may well have a long-term effect on the future
viability of our species, and may have made a significant impact on
evolution in the past."
nor to many generations ahead. But the time-scales involved in significant
changes to planetary orbits, such as that of the Earth, are not long in
comparison with the time-scales for biological evolutionary changes here on
Earth. Such orbital changes may well have a long-term effect on the future
viability of our species, and may have made a significant impact on
evolution in the past."
It's a funny thing about those time scales. Basically, the way the
astronomical experts see it these days, there are few short-term instabilities
in the Solar System for the simple reason that anything that would be likely to
happen quickly (involving the major planets at least) already did happen --and
quickly-- a long, long time ago. Here's an abstract from a review article that
addresses the long-term stability of planetary orbits:
"The physical basis of chaos in the solar system is now better
understood: In all cases investigated so far, chaotic orbits result from
overlapping resonances. Perhaps the clearest examples are found in the asteroid
belt. Overlapping resonances account for its Kirkwood gaps and were used to
predict and find evidence for very narrow gaps in the outer belt. Further
afield, about one new "short-period" comet is discovered each year. They are
believed to come from the "Kuiper Belt" (at 40 AU or more) via chaotic orbits
produced by mean-motion and secular resonances with Neptune. Finally, the
planetary system itself is not immune from chaos. In the inner solar system,
overlapping secular resonances have been identified as the possible source of
chaos. For example, Mercury, in 10^12 years, may suffer a close encounter with
Venus or plunge into the Sun. In the outer solar system, three-body resonances
have been identified as a source of chaos, but on an even longer time scale of
10^9 times the age of the solar system. On the human time scale, the planets do
follow their orbits in a stately procession, and we can predict their
trajectories for hundreds of thousands of years. That is because the mavericks,
with shorter instability times, have long since been ejected. The solar system
is not stable; it is just old!"
(from Ann. Rev. Astro., Sep 2001)
For a recent example of the manner in which chaotic dynamics can still
yield interesting information about what happened in the Solar System even
BILLIONS of years ago, take a look at this article from "Nature":
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
[ ] Mystic, Connecticut
[X] Chicago, Illinois