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Path: utzoo!watmath!clyde!floyd!harpo!seismo!ut-sally!utastro!ethan
From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.origins
Subject: retrograde moons
Message-ID: <145@utastro.UUCP>
Date: Thu, 12-Apr-84 10:31:12 EST
Article-I.D.: utastro.145
Posted: Thu Apr 12 10:31:12 1984
Date-Received: Fri, 13-Apr-84 20:44:16 EST
Organization: UTexas Astronomy Dept., Austin, Texas
Lines: 44

{To the creator of the NET, in whose image the we are made}

Don Wilson wrote

 >I just acquired a copy of the Henry Morris text titled
 >"Scientific Creationism"  (Public School Edition).
 >The book is an incredible melange of assertions, mostly
 >in areas where I personally have limited knowledge.
 >I would be interested in some intelligent comments on
 >whether these assertions have any validity. For starters:
    . . .
  >     4. The fact that one-third of the planetary satellites have
  >       retrograde orbits with respect to the rotational direction
  >      of their respective planets.

 >I hadn't realized that science was in such bad shape!  Any comments?

I have on hand a listing of the various satellites found in the solar system.
The listing is current as of mid 1981 ("Astronomy: The Cosmic Journey"
Hartmann).  There are 44 moons listed.  Of these 6 have retrograde orbits
with respect to the rotational direction of the planets they orbit.
The moons with retrograde orbits are Phoebe (Saturn), Triton (Neptune),
and Ananke, Carme, Pasiphae, and Sinope (Jupiter).  All of the retrograde
moons have large orbital tilts with respect to the equator of the planet
they orbit.  This is not true of most moons. The retrograde moons
of Jupiter are the four outermost moons listed ( 2.07e7km to 2.37e7 km).
They are all rather small with compositions similar to many asteroids.
These facts are consistent with the hypothesis that these moons are
captured asteroids.  The same seems to be true of Phoebe, although
it is rather larger than the captured moons of Jupiter.  Triton
does not seem to fit this picture.  I am not aware of any explanation
for its peculiar orbit. (I am not an expert in this field.)
For those of you who are not astronomy buffs, let me add that Jupiter
is adjacent to the asteroid belt and Saturn is the next planet out.



                     "Just another Cosmic Cowboy"
                         
                         Ethan Vishniac
                         {ut-sally,ut-ngp,kpno}!utastro!ethan
                         Department of Astronomy
                         University of Texas
                         Austin, Texas 78712