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From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.space
Subject: Re: "At the moment Uranus's south pole points..."
Message-ID: <446@utastro.UUCP>
Date: Sat, 1-Mar-86 14:24:53 EST
Article-I.D.: utastro.446
Posted: Sat Mar  1 14:24:53 1986
Date-Received: Mon, 3-Mar-86 01:23:40 EST
References: <860217-235409-1418@Xerox> <399@utastro.UUCP> <578@jplgodo.UUCP>
Organization: U. Texas, Astronomy, Austin, TX
Lines: 23
Summary: oops

In article <578@jplgodo.UUCP>, steve@jplgodo.UUCP (Steve Schlaifer x3171 156/224) writes:
> In article <399@utastro.UUCP>, ethan@utastro.UUCP (Ethan Vishniac) writes:
> > The angular momentum vector of the Earth points toward the North Celestial
> > Pole.  Put it another way, looking down on the Earth from a point above
> > the North Pole the Earth's rotation is counterclockwise.  Therefore,
> > by convention, the north pole of any planet is that pole from which
> > the rotation appears counterclockwise.
> Unfortunately, the IAU (International Astronomical Union) didn't see it that
> way.  Essentially, they decided that the north pole is the one above the 
> invariant plane (ecliptic) regardless of which way the planet spins.
> 

After much checking around here I see that I was in error.  It seems a bizarre
convention to me, but it is the IAU standard.  What tripped me up is that the
pole of Uranus is habitually described as tipped more than 90 degrees, which
is only possible if you worry about the direction of the angular momentum 
vector.
-- 
"Ma, I've been to another      Ethan Vishniac
 planet!"                      {charm,ut-sally,ut-ngp,noao}!utastro!ethan
                               ethan@astro.UTEXAS.EDU
                               Department of Astronomy
                               University of Texas