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From: throopw@rtp47.UUCP (Wayne Throop)
Newsgroups: net.origins
Subject: Re: Dinosaurs in Distress
Message-ID: <186@rtp47.UUCP>
Date: Sat, 14-Sep-85 15:31:42 EDT
Article-I.D.: rtp47.186
Posted: Sat Sep 14 15:31:42 1985
Date-Received: Sun, 15-Sep-85 23:55:40 EDT
References: <369@cornell.UUCP>
Organization: Data General, RTP, NC
Lines: 50

> ... if I understand his postings (which may not be the case) he claims
> that the gravity on earth in ancient time was felt to be lower, since
> Saturn was hanging somewhere in the sky and causing tremendous
> tidal-effects. Now, on the hemisphere closest to Saturn gravity would
> indeed be lower, but on the far side gravity would in the same manner be
> much higher (right?).

Wrong.  Tides don't work that way.  Consider a primary P with satellite
S, and on S are observers 1, 2, 3, and 4, as shown below.  We are
looking down on the north poles of P and S.

                                        1
                    P                  4S2
                                        3

Now then, what would these observers see, apparent-gravity-wise.  1 and
3 would see "normal" gravity, and 4 and 2 would see reduced gravity.
(Note that observers 5 and 6, on the poles of S but not shown, see
normal gravity also.) (Also note that I am ignoring some minor tidal
effects, and am only considering the major ones.)

This is a little surprising at first, but becomes "obvious" when one
thinks of where tidal effects "come from".  The center of mass of S is
in free fall about P.  Our 4 intrepid observers are constrained to orbit
at the same speed as S, but note that 4 is in a lower orbit, and 2 is in
a higher orbit.  Thus, 4 is going "too slow" to be in free fall with
respect to P, and thus feels a net force towards P.  2, on the other
hand, is going "too fast" to be in free fall with respect to P, and thus
feels a net force away from P.

Another way to think of it is this.  4 feels reduced gravity because P
is pulling 4 away from S.  2 also feels reduced gravity, because
(dramatic pause) P is pulling *S* away from *2*!!!

> So unless Saturn was in a geosynchronous orbit the poor dinosaurs would
> we squashed periodically, and hence succumb.

Well, no.  You apparently mean "unless Earth was either tide-locked,
or had it's pole oriented toward Saturn."

Anyway, it is correct that tidal effects could not have created a
low-gravity environment earthwide.  I think the major problem with such
tidally-created reduced-gravity zones is not one of why dinosaur
distributions don't show such patterns, but rather why Earth survived
the experience at all.  If Earth orbited Saturn close enough to reduce
gravity by, say, 1/2 due to tidal effects, the crust would be "rent
asunder", and "reduced to a state of candecence", if I have my
catastrophic idiom correct.
-- 
Wayne Throop at Data General, RTP, NC
!mcnc!rti-sel!rtp47!throopw