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Author Topic: Why if the moon is rotating can we only ever see the same side?  (Read 9678 times)

Randal

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Randal asked the Naked Scientists:
   
Firstly, thanks for saving my sanity. I'm an Aussie in China and your
podcasts are the highlight of my week.  It is great to hear English but even better to hear about interesting science stuff.

Can I ask ....why if the moon is rotating can we only ever see the same side?

I have tried to simulate this without success.

Thanks again for a fabulous program....

Randal

What do you think?


 

Offline lightarrow

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The Moon (as Earth) rotates in 2 ways: it revolutes around Earth and it spins; the 2 rotations are in the same sense and have the same angular speed, so you always see the same side.

See also:
http://www.thenakedscientists.com/forum/index.php?topic=18674.0
http://www.thenakedscientists.com/forum/index.php?topic=9511.0
« Last Edit: 07/02/2009 12:59:15 by lightarrow »
 

Offline LeeE

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The moon is 'tidally locked' with the Earth.  This happens over a long period of time where the two bodies being considered are not perfectly symmetrical i.e. mountains, canyons etc.  Gravity slows the rotation of the bodies because the gravitational force between them varies due to the tiny asymmetries and eventually stops them at the point where the gravitational forced are best balanced.  The Moon has become tidally locked with the Earth before the Earth has become locked with the Moon because the Moon is much smaller than the Earth and exerts a correspondingly smaller force upon the Earth than the Earth does upon the Moon.  Eventually though, they'll both become locked to each other, with the same faces always pointing towards each other.  That's if Sol doesn't nova before then.
 

Offline JnA

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Eventually though, they'll both become locked to each other, with the same faces always pointing towards each other.  That's if Sol doesn't nova before then.

isn't the moon moving away from the earth?
 

Offline Vern

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Yes; the moon is moving away; LeeE didn't say otherwise if you re-read the post.
 

Offline JnA

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I'm not saying that he said otherwise, just wondering about the strength of the 'locking' together if the moon is further away.

It was a question not a rebuttal.
 

Offline Vern

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I think that the same tidal forces that are causing the moon to speed up and so move farther away also cause the moon to rotate in sync with its orbit around earth. The amount that the orbital distance increases is too small to disrupt the forces that keep the moon's rotation in sync with its orbit.

So I don't think we will see the back side of the moon from earth any time soon. :)
 

Offline LeeE

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Yes, the moon is moving away from the Earth, but that wasn't part of the question.  Actually, the reason that the Moon is moving away from the Earth is because it's slowing the Earth's rotation down, so it should stop moving away from the Earth once both the Earth and Moon are tidally locked.
 

Offline Vern

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I don't think that can happen LeeE. I might have to do some research but I think that the mechanism that is causing the moon to speed up will remain until the moon escapes, or the sun ends the whole thing by its end game.
 

Offline LeeE

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It's an energy equation Vern.  The energy has to come from somewhere, in this case the Earth's rotation, and without that source of energy I don't see how the process can continue.
 

Offline Vern

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Why if the moon is rotating can we only ever see the same side?
« Reply #10 on: 08/02/2009 15:48:04 »
Yes; I agree that the energy must come from the earth's rotation. However, I think there must be enough energy in that rotation to eventually propel the moon away. But I admit; I haven't done the arithmetic. Here's a Wikki explanation of the process:

Quote from: Wikki
Gravitational coupling between the Moon and the ocean bulge nearest the Moon affects its orbit. The Earth rotates on its axis in the very same direction, and roughly 27 times faster, than the Moon orbits the Earth. Thus, frictional coupling between the sea floors and ocean waters, as well as water's inertia, drags the peak of the near-Moon tidal bulge slightly forward of the imaginary line connecting the centers of the Earth and Moon. From the Moon's perspective, the center of mass of the near-Moon tidal bulge is perpetually slightly ahead of the point about which it is orbiting. Precisely the opposite effect occurs with the bulge farthest from the Moon; it lags behind the imaginary line. However it is 12,756 km farther away and has slightly less gravitational coupling to the Moon. Consequently, the Moon is constantly being gravitationally attracted forward in its orbit about the Earth. This gravitational coupling drains kinetic energy and angular momentum from the Earth's rotation (see also, Day and Leap second). In turn, angular momentum is added to the Moon's orbit, which lifts the Moon into a higher orbit with a longer period. The effect on the Moon's orbital radius is a small one, just 0.10 ppb/year, but results in a measurable 3.82 cm annual increase in the Earth-Moon distance.[53] Cumulatively, this effect becomes ever more significant over time; since astronauts first landed on the Moon approximately 40 years ago, it is 1.51 metres farther away.
 

Offline LeeE

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Why if the moon is rotating can we only ever see the same side?
« Reply #11 on: 08/02/2009 23:29:23 »
Yup, but when the Earth has become tidally locked with the Moon and stopped revolving there won't be any tides - the bulges in the oceans will become stationary and the frictional coupling between the oceans and the seabed becomes redundant as there's no movement, apart from any weather factors.  In any case though, oceans are not necessary for tidal locking to occur; the Moon managed it without any.  While the rate of rotation of the Earth is relatively high the ocean tides will have a strong effect but as the rotation of the Earth slows down the effect of the oceans will be reduced, ultimately to the point where they play no part at all.  You need to bare in mind the speed of the water; water moving at several knots will have an appreciable effect but water that only moves a few feet per day will hardly make any difference.
 

Offline erickejah

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Why if the moon is rotating can we only ever see the same side?
« Reply #12 on: 09/02/2009 00:59:04 »
I think Chris said something about this topic in this podcast.  ::), but i can not remember which one [:-[]
 

Offline jj73

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Why if the moon is rotating can we only ever see the same side?
« Reply #13 on: 09/02/2009 17:23:53 »
Vern : The moon must be slowing down, not speeding up as it moves to greater orbital distances. If it was speeding up and also moving further out it would have left Earth orbit a long time ago.

At close distances a high speed is needed to maintain an orbit around the central body, as the orbits move further away the speed is lower. For example, Pluto's (very eccentric) orbit averages around 40 AU from the sun (about 40 times further than Earth from the sun). It's year, on the other hand, is about 248 Earth years. Distance travelled per orbit is proportional to orbital radius (2 x PI x radius), so it's overall speed is about 0.16 times Earth speed.

So, the moon is slowing down as it moves to greater orbits and as it moves out further sapping the Earth's rotational energy then the rate of movement and the orbital distance will stabilize.
« Last Edit: 09/02/2009 17:28:01 by jj73 »
 

Offline Vern

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Why if the moon is rotating can we only ever see the same side?
« Reply #14 on: 09/02/2009 20:47:30 »
I think you've got that a little bit backwards :) To reach a higher orbit you must speed up; otherwise bodies could not stay in orbit. If slowing produced a higher orbit; the slowing in the atmosphere would send stuff back out into space.

You must be thinking of the orbital period; or the amount of time it takes to complete an orbit. Going faster places an object in a higher orbit which then takes longer to complete because of the greater distance in the circle around the orbit.


Edit: The higher orbit may be slower in feet per second.
« Last Edit: 10/02/2009 13:27:34 by Vern »
 

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Offline A Davis

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Why if the moon is rotating can we only ever see the same side?
« Reply #15 on: 09/02/2009 23:44:21 »
Shrunk
The 21st centuary solution is that the Moon has no spin it's value is zero. This solution is true in spherical harmonics for n=1 and it is also true in Quantum Mechanics for the intrinsic spin of the electron in the first Bohr orbit S = 0. Looking at other solutions Mercury being the first planet solution to the sun has no spin, the first satellite of Mars has no spin the first satellite of Neptune has no spin and so on.
 

Offline Soul Surfer

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Why if the moon is rotating can we only ever see the same side?
« Reply #16 on: 09/02/2009 23:59:17 »
Vern it may be counterintuitive but jj73 is correct.  You have to do the calculation in terms of the energy of the orbit.  The orbit is higher an therefore possesses more potential energy the kinetic energy is reduced slightly but the total energy increases.

Let me illustrate it like this say you are in a spacecraft in a circular orbit around a planet and you fire the motors briefly in the direction of travel and speed up the spacecraft.  The orbit then becomes elliptical and you move further out to reach a maximum distance from the planet at the opposite position in the orbit from where you fired the motors  as you do this you are slowing down all the time.  If you do nothing you will return to the position where you fired the motors accelerating back to your final speed that was reached after firing the motors.  Now if when you are furthest away from the planet and going at your slowest speed you fire the motors again in the direction of travel to speed up a little bit  (but not to go as fast as you were in the original orbit)  it is possible to restore a slower circular orbit at the higher altitude and all you have done is make the spacecraft go faster.  The techniques of orbital energy changes are very unlike things on the earth's surface but work very accurately.

You cannot go further out in a circular orbit by accelerating a spacecraft with a single impulse you need at least two the alternative is to do it extremely slowly with a continuous tiny acceleration force but the spacecraft actually slows down because more energy is going into raising the potential energy of the spacecraft than is increasing the kinetic energy.
 

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Offline Soul Surfer

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Why if the moon is rotating can we only ever see the same side?
« Reply #17 on: 10/02/2009 00:00:59 »
Shrunk
A Davis you are talking complete rubbish.  please ignore that contribution
 

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Offline A Davis

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Why if the moon is rotating can we only ever see the same side?
« Reply #18 on: 10/02/2009 00:22:35 »
Shrunk
Maths is Science how can you ignore it.
 

Offline Vern

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Why if the moon is rotating can we only ever see the same side?
« Reply #19 on: 10/02/2009 00:36:06 »
Vern it may be counterintuitive but jj73 is correct.  You have to do the calculation in terms of the energy of the orbit.  The orbit is higher an therefore possesses more potential energy the kinetic energy is reduced slightly but the total energy increases.

Let me illustrate it like this say you are in a spacecraft in a circular orbit around a planet and you fire the motors briefly in the direction of travel and speed up the spacecraft.  The orbit then becomes elliptical and you move further out to reach a maximum distance from the planet at the opposite position in the orbit from where you fired the motors  as you do this you are slowing down all the time.  If you do nothing you will return to the position where you fired the motors accelerating back to your final speed that was reached after firing the motors.  Now if when you are furthest away from the planet and going at your slowest speed you fire the motors again in the direction of travel to speed up a little bit  (but not to go as fast as you were in the original orbit)  it is possible to restore a slower circular orbit at the higher altitude and all you have done is make the spacecraft go faster.  The techniques of orbital energy changes are very unlike things on the earth's surface but work very accurately.

You cannot go further out in a circular orbit by accelerating a spacecraft with a single impulse you need at least two the alternative is to do it extremely slowly with a continuous tiny acceleration force but the spacecraft actually slows down because more energy is going into raising the potential energy of the spacecraft than is increasing the kinetic energy.
Didn't you just say what I said? You have to speed up to get to a higher orbit; I didn't detail the second necessary burn to stabilize the higher orbit, but the stabilizing burn is also a speeding-up burn.

So the speed is faster to get to the higher orbit, the higher orbit is slower; aren't we saying the same thing :)

Edit: The original problem was: Tidal forces act upon the moon to increase its orbital speed. This causes it to move further away. Of course as it moves further away, it slows down, however, the speed increasing force is still acting upon it so this tends to stabilize its further-away orbit.

Granted; the further-away orbit may be slower in feet per second; but that wasn't the issue. The issue was whether the moon is moving further away as a result of forces tending to speed it up.

Note: jj73 has improved the original post; so that it is, as of this writing, a correct statement. :)
My original take of it was that the moon must be getting closer to earth. :) How wonderful the edit button !
« Last Edit: 10/02/2009 17:10:30 by Vern »
 

Offline yor_on

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Why if the moon is rotating can we only ever see the same side?
« Reply #20 on: 10/02/2009 15:37:20 »
I think this might work to visualize it?

If you use your fist to make a rotational movement a couple of decimeters wide and then imagine yourself standing in the center of that movement you will see that the moon (fist) should present us with different 'faces' as it turns around our globe.

The assignment for you now is to rotate that fist so that no matter where it is in that circular movement around your center it will always turn the same 'face' towards it.

You don't need to consider that our earth also is turning, as that has no meaning as long as you succeed to show the same 'face' of that 'fist/moon' at that center point (Earth).

Don't do it standing at the window though, or your neighbors will worry.
 

Offline Vern

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Why if the moon is rotating can we only ever see the same side?
« Reply #21 on: 10/02/2009 16:14:10 »
Good point yor_on :)
 

Offline yor_on

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Why if the moon is rotating can we only ever see the same side?
« Reply #22 on: 10/02/2009 17:40:01 »
Yep, I know my neighbors found me, ah, funny?

« Last Edit: 10/02/2009 18:01:14 by yor_on »
 

Offline LeeE

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Why if the moon is rotating can we only ever see the same side?
« Reply #23 on: 10/02/2009 18:18:35 »
Vern: jj73 and SoulSurfer are correct.  To go from a lower orbit to a higher orbit you need to expend energy.  However, this energy isn't to make you go faster but to take you further out, where you end up going slower.  It does seem counter-intuitive, but to go faster you need to slow down, which takes you in, which then makes you faster.

Just a couple of rough numbers:

A satellite in a circular geosynchronous orbit will travel π*d (where d is the diameter of the orbit) in 24 hours, which is

3.141592653 * 84328 = 264924 km/day = 3.06625 m/s

The moon, which has an orbital diameter of around 768806 km gives us

3.141592653 * 768806 = 2415275 km/27.321 days = 1.02318 m/s

Actually, I know the Earth's day isn't exactly 24 hours but it doesn't affect the answers significantly in terms of showing the relative speeds.
 

Offline Vern

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Why if the moon is rotating can we only ever see the same side?
« Reply #24 on: 10/02/2009 19:05:44 »
Yes, LeeE,  I see that they are both correct. Maybe I should have thought it out a little better before I said "speed up". Soul Surfer explained it correctly I think. Applying force actually does speed up an object in orbit; then it slows as it gains altitude.




« Last Edit: 14/02/2009 23:20:44 by Vern »
 

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Why if the moon is rotating can we only ever see the same side?
« Reply #24 on: 10/02/2009 19:05:44 »

 

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