Re: moon's synchronism

[Leigh Palmer <palmer@sfu.ca>]

> At 07:54 PM 3/23/98 -0800, you wrote:
> > > Now that I think about it some more the sun ought to produce a torque
> > > affecting the moon's orbital angular momentum which should cause its orbital
> > > axis to precess around a line perpendicular to the earth's orbit.  I have no
> > > idea of the period of that precession.
> >
> > That's correct. The period is about 18.6 years. The inclination
> > is appreciable, about five degrees, and the effect is to make the
> > motion of the Moon as observed from Earth a bit difficult to
> > predict by mental calculation. Another torque acting on the Moon's
> > orbit is due to the quadrupole moment of the Earth.

I should have mentioned that it is the reaction to this torque, the torque
exerted by the orbiting Moon on the Earth, that is partly responsible for
the precession of the equinoxes. The Sun is responsible for a smaller but
significant contribution to this precession.

> I should have remembered that.  I think I remember that studies of
> Stonehenge indicated that the "Aubrey Holes" were intended to keep track of
> this period to predict solar eclipses.

That's a slightly different matter. The periodicity of eclipses with
similar circumstances occurs with a period of about 18 years and 11 days,
an interval called a "saros". This periodicity is due to the near
equivalence of three different integer multiples of important lunar
periodicities:

 223 synodic months     = 6585.32 days (period between successive new moons)

 242 nodical months     = 6585.36 days (period between successive northward
                                        crossings of the ecliptic)
 239 anomalistic months = 6585.54 days (period between successive perigees)

(Data quoted from George Abell's "Exploration of the Universe", 4th ed.
The explanatory definitions are mine.)

Three saros' time is almost exactly 19,756 days, at which time the cycle of
eclipses will roughly repeat at any given geographic location. That would
be 54 years and 33 days. I think there are 54 Aubrey holes. It is solely
the coincidence between those two numbers on which the eclipse prediction
speculation is based. I consider that far too flimsy a connection to give
any weight at all. It would indeed be remarkable to find iron age knowledge
of this long periodicity, and what about those 33 additional days? This is
not what I call evidence.

Leigh