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Post by abacus9900 on Dec 3, 2013 13:36:32 GMT 1
Why is it we only ever see the same side of the Moon from Earth?
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Post by principled on Dec 4, 2013 0:11:12 GMT 1
OK Abacus, I know you've got something up your sleeve, so let's start the ball rolling with this explanation When amoeba's looked up at the nearby chunk of rock called the Moon, they marvelled at this slowly rotating planet that waxed and wained cyclically. They of course couldn't rationalise too well and so just got on with the job of dividing and evolving until they became Dinosaurs. By the time these lifted their heavy heads skywards they noted a "planet" that showed only one face to the Earth, but which still waxed and wained. Unable to talk to their predecessors, the amoebas, they were unaware that the now almost stationary planet had once rotated much faster. Unfortunately, any thoughts they had on the subject were obliterated along with them when an asteroid-or some-such- hit the ground and blotted out the Sun for a while. Some tens of millions of years later, after a few fruitless branches died out, Homo Sapien came along. Looking skywards he observed the same thing as the Dinosaurs, although at the time he didn't know it. As he developed, he took greater notice of the orbiting planet and even began to worship it. Eventually, he introduced a process called science and started to contemplate the nature of his surroundings. Apples that fell from trees onto his head now provoked thought rather than anger. He noted that seas rose and fell and seemed to be affected not only by the planet he called Sun but also by the movement of the planet he now called Moon. Putting all his thoughts together he decided that there were forces at play which affected the Earth and- he postulated- affected the Moon as well. These mutual forces of attraction he called Gravity. After much head scratching, he determined that the Moon as seen by the amoebas had been spinning on its axis but the force of gravity acted to slow it down until its rotational speed was locked to its rotation of the Earth. He also postulated that by the time the fuel on the Sun ran out, the Moon will have slowed the Earth such that its day would be 40hrs or so long. Of course unlike the amoeba and Dinosaurs, our descendants will have Google and will know what we saw and what we believed. Now, what exactly is your follow-up question?I can't wait for the discussion to begin P
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Post by abacus9900 on Dec 4, 2013 10:22:35 GMT 1
Um....I have a feeling that might not be the right answer, principled.
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Post by principled on Dec 4, 2013 11:48:54 GMT 1
Abacus OK,you got me. I'm not sure what the amoeba or dinosaur saw, but the current explanation of the Moon's rotation is based on the Earth's gravitational pull...unless you know different, of course P PS: I assume you're not talking about the fact that we actually see around 60% of the Moon's surface due to orbital irregularities.
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Post by marchesarosa on Dec 4, 2013 14:50:43 GMT 1
Because the rotation of the moon on its own axis occurs at such a slow speed relative to the earth's the same side always faces the earth See this illustration. (It is wrong, actually, in one detail, can you see why?) You will see that at the 3 o'clock position the moon is "dark" from the earth side while its illuminated side is (as always) facing the sun. Am I getting warm?
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Post by marchesarosa on Dec 4, 2013 15:03:34 GMT 1
See also "if you looked down on the Earth-Moon system from the north celestial pole, from the perspective of Polaris, the North Star, you’d see that the Moon actually does rotate on its axis. In fact, as the Moon travels around the Earth in a counter-clockwise orbit every 27.5 days, it also completes one full rotation on its axis – also moving in a counter-clockwise direction." Read more: www.universetoday.com/19699/does-the-moon-rotate
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Post by marchesarosa on Dec 4, 2013 15:32:07 GMT 1
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Post by abacus9900 on Dec 4, 2013 20:38:54 GMT 1
I think I get it, after some thought. Because the period of 1 moon orbit around the earth takes the same time as the moon rotates on its axis the two cancel out. So, on the one hand the moon is trying to rotate away from the view from earth but is counteracted by the movement of the moon's orbit around the earth thus maintaining our view of one side (I think). What a strange state if affairs.
Thanks, marchesarosa.
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