Re: Winter Solstice Sunrise

Posted by Steve Smith on Dec 21, 2012; 3:39am
URL: http://friam.383.s1.nabble.com/Winter-Solstice-Sunrise-tp7581126p7581136.html

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 <[hidden email]> 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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