Thanks Bruce!
I knew that such a collision was a leading theory and it fits the
questions I had... I didn't realize "Theia" was presumably as large as
*Mars* nor that the same theories suggest that a number of such bodies
might have existed and the Earth likely experienced a *number* of such
collisions!? The Bellbruno-Gott hypothesis regarding formation of Theia
as an accretion body at one of the Earth-Sun Lagrange points is pretty
amazing too!
The only reason I had to imagine the Moon orbit might be in the plane of
our own rotation is that if it had been formed from a proto earth-moon,
they would seem to share the same axis of (mutual) rotation? Even the
Theia hypotheses seem to beg this point?
A Theia-Earth collision, even if Theia's angular momentum vector was
aligned with the Solar System's, it's mass was order 1/10 that of Earth
(well, the present Earth?) and the resulting Moon's mass is 1/10 that
presumed for Theia, then by very loose rule of thumb, I want to guess
that the resulting earth-moon system would be roughly aligned with the
angular momentum vector of the (much) larger body... of course, the
Theia hypothesis doesn't directly account for 90% of Theia's mass... or
at least 90% of it was lost as ejecta from the collision or captured by
the Earth-body rather than the Moon-body?
I know I'm being incredibly informal in these maunderings, perhaps a
more careful analysis would offer up the answers right away. It also
seems possible that a *subsequent* event (to the Theia-Earth cum
Earth-Moon collision) adjusted the Earth's angular momentum vector...
apparently the magnitude of the earth's rotational drift or perhaps
"nutation" is not trivial.... hmmm.... time for more eggnog.
- Steve
Remember that the current consensus theory is that the Moon was
ejected from the Earth when a Mars-sized object struck the Earth.
Almost all objects in the Solar System lie in the same plane (the
"ecliptic" plane), associated with the original disk-like
concentration of material. There's no reason at all to expect the Moon
to have an orbit in our equatorial plane.
Note that the four bright moons of Jupiter, first seen by Galileo,
have orbits in the ecliptic plane.
Bruce
On Thu, Dec 20, 2012 at 7:28 PM, Steve Smith <[email protected]
<mailto:[email protected]>> wrote:
Nick/all -
While they HAVE noticed that the sunset/rise moves N and south
along the horizon in spring and fall, Few have noticed that the
moon makes that same trip in a month. Wise people have attempted
to explain this with me using a beachball, an orange a grape and a
floodlight, but the explanation still hasn't taken.
And WTF is this all about? Why does the moon orbit roughly in the
plane of the ecliptic rather than perpendicular to the rotation of
the earth?
I suppose that it is one or both of the following:?
1) The moon was not formed from the same accretion disk the
earth was?
2) The moon is large and close enough to the sun for *it* to
exert tidal forces?
But then it would seem that the *earth's* tidal forces (and it's
oblate-spheroid shape?) might pull the moon into the plane of it's
own rotation?
Hmmm... I bet someone here has already sorted all this...
- Steve
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