AURGH!! I'm still not getting this even after 5 pages.
WTHell is the dark side of the moon then and if the moon does rotate why do we never see it?
Earth's Moon
- Thread starter Orleander
- Start date
AURGH!! I'm still not getting this even after 5 pages.
WTHell is the dark side of the moon then and if the moon does rotate why do we never see it?
Janus58
Valued Senior Member
Check out these image, it is looking down on the moon's orbit from above:
The top pic is of the situation as it exists with the Moon. Note that that in order for the moon to keep one face to the Earth, it must turn as it orbits.
The second pic is what would happen if the moon didn't rotate. Note that in this case, the Moon does not keep the same face to the Earth at all times.
There is no "dark side" of the Moon, there is a "far side" of the Moon. That side which never faces the Earth.
The top pic is of the situation as it exists with the Moon. Note that that in order for the moon to keep one face to the Earth, it must turn as it orbits.
The second pic is what would happen if the moon didn't rotate. Note that in this case, the Moon does not keep the same face to the Earth at all times.
There is no "dark side" of the Moon, there is a "far side" of the Moon. That side which never faces the Earth.
Orleander, the reason is in the synchronization of orbit and spin/rotation. Here is an experiment to explain it, take a pen that has some writing on the side. Put it at the far end of you peripheral vision. Place the written material so that it is at a ninety degree angle to your line of site. At this point, the writing should not be visible to you. Now slowly move your extended hand holding the pen so that it is directly in front of you. You will notice that you are now able to read the writing on the pen. You did not move, the pen did not rotate, and you are seeing a different face of the pen because your perspective has changed. In order for you to see the same face of the moon the moon needs to rotate.
Ps I still think you are an extremely intelligent lass!
Ps I still think you are an extremely intelligent lass!
In order for you to see the same face of the pen, the pen would need to rotate as it moves around you.
Janus58
Valued Senior Member
Okay, try this. In the following animation I shaded the "planet" red and white and it rotates at the same rate as the "moon" orbits. Note that the white side of both the moon and planet always face in the same direction. It is obvious that the planet rotates, and if the moon always faces in the same direction as the planet, then the moon must also rotate as it makes its trip around the planet, even though it always keeps one face towards the planet. In fact, it must rotate in order to keep the same side towards the planet.
Okay, try this. In the following animation I shaded the "planet" red and white and it rotates at the same rate as the "moon" orbits. Note that the white side of both the moon and planet always face in the same direction. It is obvious that the planet rotates, and if the moon always faces in the same direction as the planet, then the moon must also rotate as it makes its trip around the planet, even though it always keeps one face towards the planet. In fact, it must rotate in order to keep the same side towards the planet.
Nice one, if you cant understand it from that animation you really are hopeless.
As an old teacher, I seldom give up on a "Hopeless Student."
Now imagine that the "moon" in the animation is slowing enlarging until it is equal to the "Earth" and both orbit their common mass center. Which one is not rotating? The moon? Ok, then change the labels on these identical co-orbiting objects so now, with the new labels applied, would you claim the Earth is not rotating?
Earth orbits the sun and also spins on its own axis. This is exactly what the moon does? Does that mean from the sun's point of view we have a far side that is never seen?
The moon rotates around us, but I don't see it spinning on its own axis. If it was a ball of yard attached to the earth, how would it spin on its own axis?
The moon rotates around us, but I don't see it spinning on its own axis. If it was a ball of yard attached to the earth, how would it spin on its own axis?
Janus58
Valued Senior Member
the difference is that the Earth takes one day to make one rotation and 365 days to orbit the sun, while the moon takes 27.33 days to make one rotation and 27.33 days to make ooen orbit.
no, because of the above difference in the period of the orbit and the period of the rotation.
Imagine you were standing on top of the moon in the animation (assuming that in the animation we are looking from "above" the pair looking down.
Now imagine stars in the sky. What would you see. The star would appear to travel in circles around the point straight above your head.
Now, are these stars really moving in circles, or do you see this because the moon you are standing on is spinning?
The problem you are having is that you are judging spin in reference to the line connecting the planet and the moon, when in fact, spin is judged by the stars in the sky.
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Perhaps your POV of view / disagreement is not really about the moon rotating but in the phrase "about its own axis." If some object in a box (like the animation box) has its "red side" first aligned or pointing towards "wall 1" and them a little later towards "wall 2" etc. thru walls 3 and 4 until it is again facing wall 1, I think you should agree it must have been rotatating, do you not?
I.e. is your real objection the "about its own axis" part?
If so, we need to be more clear about what is defined as "rotation" and what is defined as "orbiting."
Yes, it does.
Consider this, Orelander - you are sitting in a chair facing someone who is standing. They move around you in a circle, facing you all the time.
If they did not rotate slowly, you would also see their one side, then their back and then their other side - eventually their face again. So they MUST be turning all the time - though slowly - for you to see ONLY their face all the time.
Janus58
Valued Senior Member
Just to add to what Read-0nly said. Think of the cardinal directions as you do this. The person starts to the East of you and faces West towards you. As he circles you, he has to keep changing the direction he faces in order to keep facing you, so that by the time he is North of you he is facing South, West of you, facing East, and South of you, facing North. He is turning around his own axis facing different directions the whole time.
If he did not turn around his own axis, then he would face West the whole time. Then when he was North of you, you would see his left side. When he was West of you, you would see his back, and when he was South of you, you would see his right side. IOW, you would see every side of him as he circled you.
If he did not turn around his own axis, then he would face West the whole time. Then when he was North of you, you would see his left side. When he was West of you, you would see his back, and when he was South of you, you would see his right side. IOW, you would see every side of him as he circled you.
Yes, it does.
Consider this, Orelander - you are sitting in a chair facing someone who is standing. They move around you in a circle, facing you all the time.
If they did not rotate slowly, you would also see their one side, then their back and then their other side - eventually their face again. So they MUST be turning all the time - though slowly - for you to see ONLY their face all the time.
So if I have a rock on a piece of string and swing it round my head that rock will always have the same face towards me - does that mean it's rotating on its own axis (while anchored by the string)?
Janus58
Valued Senior Member
So if I have a rock on a piece of string and swing it round my head that rock will always have the same face towards me - does that mean it's rotating on its own axis (while anchored by the string)?
Yes, because being anchored to the string causes it to.
Now change up the experiment a little. Intead of anchoring the string directly to the rock, let's assume that there is a rod that extends through the center of mass of the rock and exends beyond the ends. You attach your string to this rod by means of a y shaped yoke. (So that if you hang the rock by the string you can spin it freely along an axis perpendicular to the string.
Further assume that there is no friction between the yoke and rod.
If you swing the rock around your head now, the rock will continue to face in the same direction at all times, and will not always have one face towards you. (just as in the example with the person and chair above.) In this situation the rock is free to do as it wants rotation wise.
When the string is anchored to the rock it is forced to keep one side towards you.
Which sort of contradicts your comment about "turning" to keep one face toward the viewer in the centre.
It can't turn on its own axis because the string anchors it.