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>Fred Hebard wrote:

>"I don't know much about astrology, but the results of at least some of
>their computations correspond to the orientation of the heavens back in
>Roman times, not now.  Thus, the first point of Aries is not now in
>Aries but between Pisces & Aquarius.  This is due to the precession of
>the earth's orbit relative to the heavens.

====================

Now for a bit of pedantry. Yes, the Earth's orbit does move, VERY slightly
and slowly, but the effect that Fred is referring to is precession of the
Earth's AXIS, not its orbit.

This is due to the ellipsoidal shape of the Earth. If the Earth were a
uniform sphere, then the gravitational attraction of the Moon (and, to a
lesser extent, the Sun) would act exactly through the Earth's centre. But
the Earth's ellipsoidal shape upsets that simple geometry. You can think of
the Earth as a sphere with a sort of "spare tyre" added round its equator.
The total attraction on that extra ring does not pass exactly through its
centre, so now there's a slight torque which is trying to align the Earth's
equator with the averaged plane of the Moon's orbit. The Moon's orbit
wobbles about a lot, but it averages to being about the same as the Earth's
orbit round the Sun. And that same orbit plane is also what the Sun's
off-centre attraction is trying to pull the Earth toward. So the Moon and
Sun combine in their action.

But the Earth, because it's spinning, is an immense gyroscope. As is the
way with gyroscopes, they resist any torque that tries to change the
direction of their spin axis. Instead, they respond to that torque by
PRECESSING. The spin axis does move, but in quite a different direction, at
right-angles to the torque, as anyone who has puzzled over the contrary
behaviour of gyroscopes soon realises. So the Earth's polar axis shifts
slowly, to make an immense circle. The direction pointed to by the North
pole will soon start to move away from Polaris (to which is still getting
closer at present) and will return to be near Polaris again in about 26,000
years. That circle is centred on a point in Draco, which moves hardly at
all with respect to the stars, the North Ecliptic Pole. That point is about
23.5 deg from the N.Pole in the sky (which, to nobody's surprise, is just
the tilt of the Earth's equator to the plane of its orbit) and has a
sidereal hour angle of just 90 deg.

I apologise to antipodeans for not giving equal prominence to their pole
(which of course moves in a corresponding way), and to anyone who considers
the above information to be particularly useless. The relevance to our
trade of celestial navigator lies in the way that those stars which are
nominally fixed in the sky have their coordinates in the almanac slowly
changing with time. Not because of movement of the stars, but because of
movement of our coordinate system, which is tied to the rotating Earth.

George.

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contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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