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Old Jun 9, 2008, 4:39 PM   #21
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Alan T wrote:
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As I hope you know, I'm amenable to being told to shut up when you've had enough !
Alan, I was not trying to end this thread at all, on the contrary, I would like to know if there is some kind of real explanation on the tilting moon. I truly do appreciate all thediscussion that has gone on in this (and other threads).

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Old Jun 9, 2008, 6:26 PM   #22
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The Oregon Museum of Science and Industry (OMSI) has an excellent planetarium. I will go there tomorrow and see if I can get some answers.

Cal

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Old Jun 9, 2008, 7:39 PM   #23
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OOooo, I missed this thread earlier. This is right up my alley, so to speak. I teach science and have been a avid amateur astronomer since my teens.

OK, Here we go. We all know the the motion of the sun and moon through the sky is a function of Earth rotating beneath them. Take your right hand and arm and extend them across your body to the left. Keep your arm straight. Your right thumb should be pointing down. Now, keeping your right arm straight wave it in a slow arc up and over to the other side of your body, to the right side. Ah, now your thumb should be pointing up. I know, I know, your arm and hand (representing the moon) moved not your body (representing the Earth). OK fair enough, BUT.... imagine if you could detach your arm and hand and keep them perfectly still while someone rotated you instead. The effect would be the same as the earth rotating under the moon. With a 180° rotation the thumb would move through the "sky" and it's orientation would change with respect to body, exactly the way it does when you wave you arm across your body slowly. Thus, as the moon appears to move through the night sky it's orientation with respect to the horizon changes.

Any questions? I hope that was clear enough,

Cheers,

Steve
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Old Jun 9, 2008, 8:05 PM   #24
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Ok let me get this straight. The rotation of the earth is not at right angles to the moon. As the moon moves around the earth the rotation of the earth is on an angle which makes it appear the moon is rotating, which we know(?) it doesnt.
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Old Jun 9, 2008, 11:37 PM   #25
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Bynx wrote:
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Ok let me get this straight. The rotation of the earth is not at right angles to the moon. As the moon moves around the earth the rotation of the earth is on an angle which makes it appear the moon is rotating, which we know(?) it doesnt.
All I am trying to say is that the reason the moon appears to change orientation as it moves through the sky is becuase it is in fact not rotating from our point of view on the earth. It take 27.3 days for the moon to rotate once. In that same 27.3 days it orbits the earth once. This keeps the same side of the moon always facing the earth. You are correct, the rotation of the earth is not at right angles to the moon. It's more like 23.5 degrees give or take 5. As the moon slowly, very slowly (every 27.3 days) moves around the earth the much faster rotating earth (once every 24.25hrs) gives the perseption that the moon is moving across the night sky. As it does so the orientation of the moon changes 180° from moonrise to moonset.

Hold a tootsie Pop by the stick with the pop tilted to the left and the stick parallel to the ground. Now rotate your wrist so that the pop make a clockwise arc 180° and stop when the stick is again parallel to the ground. Notice how the oreintation of the pop changes as it goes through the 180°. The moon does the same thing. With out the stick, of course. :blah:

I hope you can follow my mental visuals. This mental exercise has done me good, I hope the rest of you could follow. If not keep asking, I'll do my best.

Steve



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Old Jun 10, 2008, 8:17 AM   #26
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Thank you Steve. That make perfect sense to me. That was a very good explanation of the principal here also

Mugmar
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Old Jun 10, 2008, 10:51 PM   #27
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Great! An excuse to buy Tootsie Pops! Should I buy yellow ones or brown ones?:-)

Good explanation.

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Old Jun 12, 2008, 1:38 AM   #28
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nhmom wrote:
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Great! An excuse to buy Tootsie Pops! Should I buy yellow ones or brown ones?:-)

Good explanation.

Patty
You need an excuse?:? I buy them by the box to avoid having to make that decision. :lol:
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Old Jun 12, 2008, 4:50 AM   #29
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I tried to draw some diagrams to illustrate Steve's solution, but decided my draughtmanship within the required timescale wasn't going to be up to it. But I thought of the following alternative verbal description.

First imagine that the Earth-Moon system is very simple, and that the Moon orbits in the plane of the equator. It's then obvious that you'd get a different view of it; you'd be seeing it at a different angle, depending on where you were on the planet. You'd get the extremes of the differentviews from our North and South poles.

Now tilt the Earth's axis of rotation towards the Moon. Any point on the Earth will get its own two extremes of the angle of view on occasions, when it's at the 'top' when facing the Moon, and when it's at the 'bottom' of that point's rotational path.

This is then further complicated by the facts I just looked up in Wikipaedia...

http://en.wikipedia.org/wiki/Moon_%28Earth%29

...that the Moon's orbit round the Earth is inclined at....

"5.145° to the ecliptic[1][/suP]
(between 18.29° and 28.58° to Earth's equator).

The Wikipagesays...

"Small variations (libration) in the angle from which the Moon is seen allow about 59% of its surface to be seen from the earth (but only half at any instant)".

There is also a rather beautiful animation of the librationon the page, nicely showing the dark 'seas', which intrigued us in Mugmar's first shots,wafting gently up and down as the angle of view changes.

Enjoyeating your sweeties as you try it out!
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Old Jun 13, 2008, 1:16 PM   #30
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Thats a great simlulation of the monthly lunar cycle. Thanks Alan for posting it.
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