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Astronomy question
Posted: 30 December 2007 12:32 PM   [ Ignore ]
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This is one of the things my son want to post after he joined, but couldn’t.

We are in a debate about the moon.  He insists that the dark side of the moon (not the far side, but the dark side- so he says are different) will never be seen by the earth.  Thing is, and this is my argument, the moon not only revolves around the earth, but also spins on it’s own axis much like the earth and everything else.  I say, theoretically, there is a probability of one in so many changes that the opposite side of the moon (the side that is dark right now) will face the sun as well as the earth.  He insists it will never happen due to the differences in how fast the earth and the moon spin on their axis.  “Never” is a big word even for science, IMO. That which is in the dark eventually faces the sun, just as the earth does, and therefore becomes light, even if it does spin a little slower, so eventually the earth will see the side that was dark, but the probability of 1 in X chances that the side of the moon that is dark now will eventually face the earth and the sun at the same time.

So the question that he wanted to propose, or rather refute me on, is will the dark side ever face the earth (or something to that affect)?  He’s not here right now to get his exact words he wanted to say, but that is basically our dispute/debate.  He thinks I’m refusing to believe a scientific fact, but there is, IMO, a low probability.  Sounds like a sophmoric question/debate, but that was one of the reasons he wanted to join so we can put an end to our little debate.  LOL

Now, I will admit astronomy is not my forte, but logically there would seem to be a probability factor, in this case, as well as my opinion.  He infactically says I am wrong and my theory is an impossibility proven by scientists.  I don’t know if scientists can disprove such a statistical probability, not yet at least.

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Mriana
“Sometimes in order to see the light, you have to risk the dark.” ~ Iris Hineman (Lois Smith) The Minority Report

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Posted: 30 December 2007 01:40 PM   [ Ignore ]   [ # 1 ]
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Actually, I believe that the moon rotates in synchrony with the earth, so we do always see the same side of it. HERE and HERE are a couple of discussions of this. The dark side is that facing away from the Sun, and the far side facing away from the earth, so these are not the same.

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Posted: 30 December 2007 02:18 PM   [ Ignore ]   [ # 2 ]
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The moon doesn’t spin on its axis. This is why we never see the other side.

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Posted: 30 December 2007 02:27 PM   [ Ignore ]   [ # 3 ]
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George - 30 December 2007 02:18 PM

The moon doesn’t spin on its axis. This is why we never see the other side.

Actually the fact that we always see the same side of the moon means it must spin on its axis. (It spins about the earth/moon barycenter).

The moon has been “captured” by earth’s gravity to the extent that its spin and earth’s are completely synchronized. IIRC eventually earth’s spin will synchronize with the sun so it will spin once per year.

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Posted: 30 December 2007 02:41 PM   [ Ignore ]   [ # 4 ]
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I don’t understand then. I thought it rotates around the earth like a Marry-Go-Round. I just tried it with two balls: if I have one ball spinning on its axis, the other one cannot spin on its axis if I want to keep seeing only one side. (This is crazy: I am 36 years old, I should know if the moon rotates… grrr)

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Posted: 30 December 2007 02:47 PM   [ Ignore ]   [ # 5 ]
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The Moon rotates on its axis, and revolves about the Earth.

At some point in the misty past, its period of rotation was faster than its period of revolution. (Earth’s rotation is around 365 times faster than its period of revolution, altho’ you may not have thought of it that way.) Now the Moon’s rotation is just as long as its revolution. This may sound strange, but to say it has no rotation would ignore that at some point the dark side of the Moon is facing one quadrant of the Solar system, and then facing the next quadrant, then etc til it comes back to facing the first quadrant again. (suppose a squirel on a tree, avoiding a man who chases it around the tree. The squirrel faces N, then E, then S then W then back N. The squirrel *is* rotating about its little squirrel spine, in comparison to the field the tree is standing in.)

Since the Earth and Moon affect each other because of their mutual gravity-fields, and b/c of the Moon’s momentum, both the Earth and Moon have slowed their axial rotations; but the Moon being smaller, it slowed down more rapidly. Now it is tidally locked, and further, this is now permanent - it’s a ‘low spot’ energetically. There is no chance at all that we’ll Pink Floyd’s giant prism from the ground here in the future, given our present situation. If perchance some massive body were to disturb this equilibrium in the right way, yes, the Moon might be given enough positive or even backwards spin (backwards relative to the counter-clockwise revolutions and rotations of all planetary bodies and most satellites). But given the planetary bodies we have now, and barring such utterly unpredictable events, you’re stuck with the Man in the Moon. Once a body ‘falls into’ being tidally locked, it doesn’t ever slow its rotation even more to end up rotating slowly backwards.

Some far distant day, the Earth’s rotation too will slow enough to become tidally locked with the Sun, and one face will forever be turned to the Sun.

Here are favorite places of mine, from NASA:

Astronomy picture of the day: http://antwrp.gsfc.nasa.gov/apod/astropix.html
Earth Observatory: http://earthobservatory.nasa.gov/

Sincerely,

Kirk

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Posted: 30 December 2007 02:47 PM   [ Ignore ]   [ # 6 ]
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dougsmith - 30 December 2007 02:27 PM

IIRC eventually earth’s spin will synchronize with the sun so it will spin once per year.

Does this mean that one side of earth will also be always dark?

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Posted: 30 December 2007 02:48 PM   [ Ignore ]   [ # 7 ]
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Nuts. As usual, Doug’s answer is shorter than mine.

(Lathering up; picks up Ockham’s razor) Darn, my beard is tough. Tough as Plato’s.

Kirk

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Posted: 30 December 2007 02:52 PM   [ Ignore ]   [ # 8 ]
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George - 30 December 2007 02:47 PM

Does this mean that one side of earth will also be always dark?

Yes. (Though this isn’t true of the moon, so “also” is the wrong word to use. In fact there is no “dark side of the moon” as in the Pink Floyd album. All there is is a side of the moon that never faces earth).

But it won’t happen for several billion years, IIRC, so nothing to worry yourself about just now.

wink

No worries, ITG—anyhow Occam’s Razor should only be used to eliminate the unnecessary!

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Posted: 30 December 2007 03:11 PM   [ Ignore ]   [ # 9 ]
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I am going insane!!! shock I keep trying this with the two balls: the moment I start spinning the “moon ball,” it’s dark side eventually catches up to face the “earth ball.” What am I doing wrong? Help!

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Posted: 30 December 2007 03:12 PM   [ Ignore ]   [ # 10 ]
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Well, there is a “dark side of the moon,” it’s just not always the same portion of the surface. Some portion is always in darkness just as some portion of the earth is. But the “dark side” changes continuously while the side facing the earth is always the same.


George,

Try this. Ignore the fact that the earth rotates, so hold that ball still (just set it down on a table or something); walk around it with the other ball not turning at all relative to the room you’re in. You will see that the side of the ball you’re walking with that faces the earth ball will gradually change. Now walk around the earth ball again but deliberately keep the same face of the moon ball pointed towards the earth ball at all times. You will find this requires you to turn the moon ball slowly, so that when you have walked halfway around the earth ball, the moon ball will have complete 1/2 rotation (relative to the room). Does this help?

[ Edited: 30 December 2007 03:16 PM by mckenzievmd ]
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Posted: 30 December 2007 03:16 PM   [ Ignore ]   [ # 11 ]
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But Doug said that “there is a side of the moon that never faces earth.”

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Posted: 30 December 2007 03:18 PM   [ Ignore ]   [ # 12 ]
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But Doug said that “there is a side of the moon that never faces earth.”

And he’s right. Try the experiment above and I think it will be clearer. We tend to talk about the “dark” side of the moon casually as if it were the side facing away form earth, but it’s not. When the dark side of the moon faces the earth, that’s the New Moon. When the light side faces the earth that’s a full moon. In between are the other phases.

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Posted: 30 December 2007 03:23 PM   [ Ignore ]   [ # 13 ]
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George - 30 December 2007 03:16 PM

But Doug said that “there is a side of the moon that never faces earth.”

That’s true, but think of it as the *far* side of the Moon. We’ll never see that side from Earthside. (Maybe Pink Floyd meant it’s ‘dark’ to our knowledge.) Think of the Sun, the the Moon, then Earth in that order - that’s either a new Moon or, if they’re all lined up just right, a solar eclipse. The far side of the Moon - which we’ll never see here - is getting the full rays of the Sun.

So ‘far side’ is less misleading than ‘dark side’ of the Moon. But ‘Far Side of the Moon’ sounds like it was composed by Gary Larson, not Pink Floyd.

Kirk

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Posted: 30 December 2007 03:37 PM   [ Ignore ]   [ # 14 ]
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Yes, I see it now.  red face  red face I was rotating the ball as I was moving it around the earth ball without noticing it. Thank you for your patience, Brennen. (Now I only have to explain to my wife, who walked in on me while running around a soccer ball in my office, that I am not going insane.)

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Posted: 30 December 2007 03:44 PM   [ Ignore ]   [ # 15 ]
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Thanks, Kirk. I now understand it. I am slow, but eventually I’ll get the point. (Except maybe for compatibilism in free will, where no soccer balls will help me.)

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