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Re: Solar System's rotation



At 08:00 PM 9/9/01 -0400, Cooper John N wrote:
Since the Milky Way runs roughly north south I gather the axis of
rotation of the earth and roughly the axis of rotation of the solar
system is roughly in the plane of the galactic rotation.

Warning to readers: That statement is hard to parse, because it speaks of
the angle between an axis and a plane. It would be better to compare
apples to apples (plane to plane, or axis to axis).

Also, the premise is overstated. Saying the milky way runs north/south or
even roughly north/south is an overstatement. On a November evening when
Andromeda is high overhead, the Milky way runs pretty much smack east/west,
from the eastern horizon straight across to the western horizon.

More generally, according to the star chart I've got here, the galactic
equator crosses the ecliptic at very nearly a 60 degree angle. That's
quite a poor approximation to 90 degrees.

To say the same thing the other way, the galactic axis is tilted very
nearly 60 degrees w.r.t the ecliptic axis.

Suppose we run a simulation, creating multiple "model universes". Without
loss of generality, let the North Ecliptic Pole be the reference point on
the projective sphere, and ask how far away the Galactic Pole falls on this
sphere. If each piece of area is equally likely, then about half the time
the GP will be mis-aligned by more than 60 degrees, and about half the time
by less. (The median is more than 45 degrees, for the same reason that
there's more area in the tropics than in the arctic regions.) From this we
conclude that the observed mis-alignment is about typical of what you would
expect from a random process.

We can agree that there is a huge mis-alignment between the galactic axis
and the ecliptic axis. We should just say it that way, and not try to
derive that conclusion from inaccurate statements about how the milky way
appears.

There's no reason to assume the plane of the orbits of other planetary
systems in the galaxy would be roughly parallel to the galactic plane of
rotation, is there?

Indeed not. Ours isn't parallel. Why should theirs be parallel? Ours is
off by 60 degrees. Why should theirs be off by less?

By a slightly more complex argument, we can answer a different but related
question: We don't expect other planetary systems to be aligned with our
system.

Reason: If the galactic pole were aligned with the ecliptic pole, one
might begin to wonder whether other systems were aligned the same way to
the same reference. If you say they are aligned, there is only one way
they can be aligned. But once they are allowed to be mis-aligned relative
to the galactic reference, there are many many inequivalent ways for them
to be mis-aligned, and there's no reason to expect other systems to have
the same mis-alignment.

Happy families are all alike;
every unhappy family is unhappy in its own way.
http://www.ccel.org/t/tolstoy/karenina/karenina.txt