# Earth's Moon

Discussion in 'Earth Science' started by Orleander, Jul 9, 2007.

1. ### SciencelovahRegistered Senior Member

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4,349

I think it's more or less like this:

@ Janus58: I hope you dont mind I grab your animation

Last edited: Aug 12, 2007

3. ### Janus58Valued Senior Member

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1,852
No, it turns because of the string. One rotation on its own axis per revolution around your head. The string just prevents it from rotating at any different rate than this one to one ratio.

I think the problem some people are having is in separating the concepts of orbital motion from the rotation of the object around it own axis.

Orbital motion is simply the path the center of gravity of the moon takes around the planet.
Rotational motion is how the moon changes the direction it faces over time.

The two motions are completely separate and independent of each other.

It seems that some seem to think that if we say one object "orbits" another, that keeping one side always facing the object it orbits is assumed to be part of its orbital motion. As if this is the natural state of an "orbit". It is not. There is no connection between an object's orbit and the direction it faces.

When we do see such a correlation, it is because outside forces (such as tidal forces, or the tug of a string) have acted on the object to cause it.

5. ### OliHeute der Enteteich...Registered Senior Member

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11,888
Ya, but it's turning on an axis at the end of the string (i.e. your head) not its own axis.

7. ### Janus58Valued Senior Member

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1,852
Two separate motions measured with respect to two different reference points, and it's doing both

This is an object traveling around a central axis and not turning on its own axis.

And the one in this post:

http://sciforums.com/showpost.php?p=1504422&postcount=108

is doing both.

Last edited: Aug 13, 2007
8. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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23,198
If you were an ant on the string tied ball, which is orbiting a second larger object, (ball always presenting only one side to larger object of course) and smart enough to note in which direction the string was pointing out into space, then you would see that "sting extended point" in the heavens moving around large circle amoung the stars. Thus the string tied ball is rotating.

MORE GENERALLY:

Any motion of any ridged body can be considered to be composed of two distinct types of motion: Rotation and Translation.

A vector (or speed & direction if you are not familiar with vectors) is associated with both and these vectors can change with time, but at any given instant, their directions are uniquely defined.

EXAMPLE:
Translation part:
If you place yourself on the rigid object and have the normal egotistical attitude then you do not consider that you are translating. For example while sitting at your desk you actually have a very complex translation with the sun thru space (in essentially a constant direction and speed, neglecting the small wobble Jupitor makes in the sun's trajectory thru space) and you are also in near circular orbit about the sun, so this is the next major part of your translation motion but now the direction of this slower component is not constant. It is constantly and steadly changing (after one year it will start repeating). There is still a third and minor component of your total translation, due to fact your desk is not at the mass center of the Earth (actually the Earth moon system as that is what is orbiting the sun, not the Earth).

Rotation part:
Even though you sit facing your desk and looking out the large window on the other side of the desk for 24 hours without thinking that you are rotating you are in fact rotating. For example, you see the sun rise and about half a day later you see it set thru the window. Now for most of man's history it was thought that you were not rotating and that the sun was going around the Earth, but I am assuming you know better than ancient man did. Thus, you are rotating even if sitting at your desk with both your hands glued to the desk. Your rotational rate is 15 degrees/ hour.

POINT of this example:
Is that you should "look to the heavens", not your local enviroment, to tell if you are rotating or not.*

(Leaving the example and returning to the ant on the ball):

Yes, it sure seems to the ant (from his local obsservations) that he is not rotating, but this is a very intelligent ant. He knows how to tell if he is rotating or not. He saw the string sweep out a 360 degree circle in the heavens while he made one orbit around the "larger object" so he understood that despite the local appearances, he was rotating.

Please be smarter than ancient man and as smart as that ant.

PS to Janus58:
For god's sake let them continue to think the moon is orbiting the Earth. - I am not up to showing that is an erroneous concept. (One I held until you explained the error to me. - thanks again for doing so.)
--------------------------
*Initially I had "must" instead of "should" in this sentence and only changed as it is not technically true with "must." (If you could precisely measure gravity at all points of interest and exactly determine the shape of a liquid surface, you could also tell if you were rotating, even if you could not observe the heavens. - Think what shape water takes in a bucket if it is rotating water to understand the point.)

Last edited by a moderator: Aug 12, 2007
9. ### OrleanderOH JOY!!!!Valued Senior Member

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25,817
Hallelujia and praise be to inzomnia!! I THINK I GOT IT!! In my head, I had a different axis. So sometimes the man in the moon is upside down?

10. ### SciencelovahRegistered Senior Member

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4,349
Eheheh, not really, actually I heard the angle should be somewhat tilted.

I dont think so. Its like on earth, there is this gravity that help us to stay not upside
down. But I am not sure though, what if we are in the middle/core center inside
the moon, maybe it could be at some point upside down, lol.

11. ### Janus58Valued Senior Member

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1,852
Sorry, but no. The moon's axis does not point along the line joining it with the Earth. Besides with such an arrangement, the axis could only point along that lie at two points of the orbit, As the axis would always point in the same direction relative to the image no matter where it was relative to the Earth.

Here's another try at a animation to get the idea across. This one shows the Libration I talked about earlier.

In it, the planet still makes one rotation per orbit of the Moon. The Moon also makes one rotation per orbit. (note that the Planet's red and white sides always face the same way.

The orbit however is now elliptical. In this case the moon travels faster in its orbit when it is nearer the planet then when it is further away. Notice how the moon does not always keep the same point facing the Planet. (the part of the moon visible to fromt he planet is shown as brighter in the image.)

IOW, both the moon and planet rotate around their axie at a constant rate while the orbit varies in speed.

P.S. the orbital speed to distance ratio does not match that of a actual satellite, and is only a rough aproximation. I just didn't want to spend the extra time fitting in the proper equations, since it did not effect the principle demonstrated.

12. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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3,181
I think you've got to specify which frame of reference you are speaking of, and rotating relative to what.

For example, an astronaut sitting on the surface of the moon and looking at Earth directly overhead would correctly conclude the moon is not rotating on its axis relative to the Earth. The Earth would stay in essentially the same location throughout the month. The astronaut would see the Earth spinning on its axis. The astronaut would see the sun move across the horizon, taking 28 days to come back to the same location in the sky. The moon is rotating relative to the sun and the stars, but it is not rotating relative to the Earth.

Next, consider that we changed the orientation of the moon's axis exactly 180 degrees. North was now south, etc. The astronaut sitting on the moon would see no difference in his observation of the Earth. An observer on Earth would see no difference in his view of the tidally locked side of the moon. But the astronaut sitting on the moon would see the sun reverse its direction across the moon's horizon. Again, the moon is rotating relative to the sun and the stars, but it is not rotating relative to the Earth. It's a matter of frame of reference as to whether the moon is rotating or not.

13. ### Janus58Valued Senior Member

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1,852
This type of change of refernce doesn't address the situation at hand. While the Astronaut might not consider himself rotating relative to the Earth, neither does he consider himself revolving around it.

The question at hand is:

From a reference frame in which the moon is considered as revolving around the Earth, is it also rotating around its own axis? and the answer is yes.

14. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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3,181
Janus58,
That is actually a strawman argument, Janus58. The astronaut sitting on the moon is not rotating relative to the Earth. He is revolving around the Earth, which he can easily determine by looking at the stars' motion. An anology would be our familiar merry-go-round oft used in science. A person sitting on the edge of revolving merry-go-round would represent the astronaut on the moon. Both are revolving around a central axis. Do we normally state the person on the merry-go-round is rotating on his axis (or ass,

)? In the frame of reference of a bystander not on the merry-go-round (consider one on the sun for the moon), the bystander will see all sides of the revolving rider, just as an observer at the sun would see all sides of the moon. The reason those observers see all sides of the rider and the moon is due only to their revolving around a distant axis, not because they are rotating on their own axis. I know you are explaining it as is written in textbooks and online physics sites, but the part about the moon rotating on its own axis is a bit a of a semantics mistake, I believe. The moon does rotate in a global frame of reference, but it does not rotate about its own axis because it is tidally locked with the Earth.

15. ### OphioliteValued Senior Member

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9,232
Wrong, wrong, wrong. oh so wrong.
Really nothing more need be said.

16. ### OrleanderOH JOY!!!!Valued Senior Member

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25,817
why does the white side of the earth never see the moon?

17. ### Janus58Valued Senior Member

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1,852
The animations aren't meant to exactly reproduce the Earth and Moon, but a simular situation where the planet makes one rotation in the same time as the moon makes one orbit. I did this for two reasons.

1. If I reproduced the actual rotation of the Earth, it would have to rotate 27.3 times for every orbit. In essence, it wouldn't be a red and white ball but a pink blur of one.

2. The rate at which the planet rotates doesn't matter in the situation being discussed, so I picked a convenient one, one which showed how the directions which the rotating planet and the moon faced always stayed the same. It was an additional visual aid to try and get the concept across.

18. ### Janus58Valued Senior Member

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1,852
Hmm. how to put this, oh yeah,

I know it's a but flippant, But any time someone says that they are right and the textbooks wrong, I think its deserving.

19. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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3,181
Janus58,
I know it is a bit flippant, but appeal to the youtube is not an argument. Point out my mistake, if you can, and explain the definition of 'tidally locked' while you are at it. An example of rotation would be the gondola of a farris wheel. The gondola rotates about its axis within the frame of the farris wheel as the farris wheel revolves around its own axis. Or do you claim the gondola does not rotate on its axis? I also recall you stating that the moon did not orbit the Earth, but it was an illusion caused by the moon's and the Earth's mutual orbit of the sun. Textbooks state that the moon orbits the Earth well within the Earth's Hill sphere, thus it orbits the Earth. Were you stating the textbooks are wrong, or were you wrong when you made the argument?

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Egad - where in the world did THIS come from???

You took something very, very simple and turned it into a complex, convoluted mess absolutely full of errors!

I'd suggest you study a bit more on this topic. In fact, you can get what you need to know simply by re-reading all the valid posts in this thread.

21. ### AtomRegistered Senior Member

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928
thee Moon is thy proof ov God.

How wonderful he didst create this masterpiece.

Look upon its size...it doth fit the Sun like a well placed coin during an eclipse.

Its cycle doth coincide exactly to a female period.

It doth show its only side in perfect confluence with thee exact synchronocity ov rotation.

It doth then result in thy biggest selling albumof all time...Thee Dark Side of thee Moon.

Moon...we worship thy female goddess nature.

All that you touch
All that you see
All that you taste
All you feel.
All that you love
All that you hate
All you distrust
All you save.
All that you give
All that you deal
beg, borrow or steal.
All you create
All you destroy
All that you do
All that you say.
All that you eat
And everyone you meet
All that you slight
And everyone you fight.
All that is now
All that is gone
All that's to come
and everything under the sun is in tune
but the sun is eclipsed by the moon.

"There is no dark side of the moon really. Matter of fact it's all dark."

22. ### Janus58Valued Senior Member

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1,852
It was not meant to be an argument, just a statement.
OK
Let's start here. The astronaut cannot determine that he is revolving around the Earth by looking at the stars (excluding using parallax.). He can conclude that the moon is rotating on its axis or that that the stars are revolving around him. (in which case, the Earth is simply suspended overhead). The reason being that the moon's orbital path around the Earth has nothing to do with a moon-based observer's perception of the stars motion. A case in point, as I already have pointed out, the Moon has an eccentric orbit. and travels around the Earth at different speeds during its orbit. An observer on the moon would not see the motion of the stars vary to match however.
He could conclude that he was revolving around the Earth, In which case, he would have to automatically conclude that the moon was also rotating in order to keep the Earth in the same place in the Moon's sky. (actually oscillating around a point in the sky due to the varience of orbit.)
No, you see what you see because the rider both revolves around the center and rotates on it own axis. Revolving around an axis does not automatically imply that the object changes the direction it faces as it does so. If the carousel horse and rider were not rigidly tied to the carousel, but rather attached by a pole passing through their Cog though frictionless bearings, then the observer would not see different sides of the rider and he revolved around the center, bt rather the same side at all times.
Tidal locking refers to a situatition where there is a correspondence between the period of an object's rotation and the period of it orbit due to gravitational interaction between the two bodies (in the case of the Moon. 1:1). It [i\]does not [/i] necessarily mean that it keeps the same face facing the object it orbits. Cases in point are the moon which exhibits libration and Mercury which has a 3:2 tidal lock with the Sun, where it rotates 3 times for every 2 orbits.
You can't have it both ways. Either the gondola rotates around its axis within the frame of the ferris wheel and within said frame The ferris wheel does not rotate around its axis, or Or the Ferris wheel rotates around its axis, while the gondola travels around this axis, while not rotating around its own. You are trying to eat your cake and have it too.
Actually, I said that it could be argued that the Moon primarily orbits the Sun, and thus secondarily orbits the Earth and that thus the Earth-Moon system could be considered a double planet rather than planet-satellite system. The recent definition of planet however appears to preclude this designation.
The particulars of the Earth-Moon-Sun orbital characteristics can be found in any good textbook,(One I have discusses both the Moon's Geocentric and Heliiocentric orbits). and some textbooks might even have broached the subject of the Earth-Moon system definition.

OTOH, no text book would allow for simultaneously considering the Moon revolving around the Earth, and not rotating around its own axis as you have tried to do.

23. ### James RJust this guy, you know?Staff Member

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30,862
I'd just like to note here that inzomnia's picture is wrong.

The Moon's axis of rotation is parallel to the Earth's axis; the Moon's axis does not point radially towards the Earth as in the diagram.

If inzomnia's picture was correct, then we would not always see the surface features of the moon with the same orientation. Instead, we'd see the moon appear to rotate as we looked at it in the sky - sometime the "man in the moon" would be the right way up, sometimes upside down, sometimes sideways, from our point of view on Earth. Obviously, that is not what we see from a fixed point on the Earth's surface.