# Why do we only see one side of the moon?

#### Azrioch

My friend who is taking an Astronomy course said that the Moon and the Earth rotate at the exact same speed because the Earth and Moon were both hit by something at the same time... or something to that effect. It didn't really make logical sense to me.

Does anyone know why we only see one side of the moon?

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Greetings!

I think he has something wrong there as well. The moon and the earth do not rotate at the same speed. Rather, the moon rotates at the same speed that it revolves around the earth. This is because it is tidal locked in the earth's gravity.

This site has some good info for you:
http://starryskies.com/The_sky/events/lunar-2003/eclipse9.html

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#### Phobos

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Welcome to Physics Forums, Azrioch!

Like Brad_Ad23 said, the Moon and Earth don't rotate (spin on their axis) at the same speed.

Visual aid...Take a quarter and a penny (or whatever 2 Italian coins that have faces). Lay them flat and move the penny around the quarter (orbit) such that the face on the penny is always facing the quarter. See that the orbital period of the penny (once around the quarter) matches the penny's period of rotation (one spin on its own axis). That's essentially the earth-moon situation.

As explained by the links provided, this is due to gravity which pulls the Earth and Moon into a slightly elongated shape (like how the moon rises the ocean tides on the Earth...this works on rock too just to a much lesser extent). As that elongated shape rotates forward (with the rotation of the planet/moon), there is a slight braking action as the gravity of the other pulls back on that raised portion. The Moon's rotation has been slowed to a point where it's rotation speed matches it's orbital period. So it maintains the same face toward the Earth. But the moon still rotates with respect to the sun (as you see from the quarter/penny example).

http://www.seds.org/billa/tnp/luna.html [Broken]

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#### AndersHermansson

If the same side of the moon is always facing earth then it is not rotating around it's own axis at all. According to GR, the moon is following a "straight path" around the Earth.

#### chroot

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Originally posted by AndersHermansson
If the same side of the moon is always facing earth then it is not rotating around it's own axis at all. According to GR, the moon is following a "straight path" around the Earth.
You're an idiot.

- Warren

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Dearly Missed
Originally posted by AndersHermansson
If the same side of the moon is always facing earth then it is not rotating around it's own axis at all. According to GR, the moon is following a "straight path" around the Earth.
If you face a lampost and walk around it, always facing toward it, you will find that the landscape behind it rotates the same way it would if you stood still and turned around. In fact you will successively face every point of the compass. To put it shortly, you will have turned around. And that's what the moon does vis-a-vis the earth.

#### russ_watters

Mentor
Originally posted by chroot
You're an idiot.

- Warren
So unnecessary.[zz)]

#### subtillioN

Originally posted by chroot
You're an idiot.

- Warren
Hi Warren,
How are you doing, I mean really?

You seem like such a nice guy and your comments demonstrate your absolute brilliance in deconstructing the errors you find in all of these silly questions!!! ((who needs questions anyway when one has all the answers?... oops!))

Keep up the good work and maybe some day you will win the Nobel Prize for insightful criticism!!

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#### chroot

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Hell, I'd be happy with the Nobel in physics, personally.

Some questions beg answers; some statements beg intelligent counterpoint. Others... simply aren't worth it.

- Warren

#### subtillioN

Originally posted by chroot
Hell, I'd be happy with the Nobel in physics, personally.

Some questions beg answers; some statements beg intelligent counterpoint. Others... simply aren't worth it.

- Warren
And so why make the effort to belittle someone? Oh yes, it is to state your superiority for everyone to see. What else could it be?

#### chroot

Staff Emeritus
Gold Member
Originally posted by subtillioN
And so why make the effort to belittle someone? Oh yes, it is to state your superiority for everyone to see. What else could it be?
Why yes, you're correct. I am, in fact, better than you. Good of you to see it. :)

- Warren

#### subtillioN

Originally posted by chroot
Why yes, you're correct. I am, in fact, better than you. Good of you to see it. :)

- Warren
so your ego escapes into the open...

#### chroot

Staff Emeritus
Gold Member
Originally posted by subtillioN
so your ego escapes into the open...
Was it ever contained???

- Warren

#### subtillioN

Originally posted by chroot
Was it ever contained???

- Warren

ok so it has taken over your rationality...which apparently is not hard to do.

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In a very weak sense you are correct. Orbits are merely the paths of least action in a warped spacetime, however, it does not necessitate an object in orbit not rotate. See for example: The earth rotates as it revolves around the sun.

It is merely that at certain distances, gravity tidal locks objects into a rotational period equal to the revolutionary period.

#### kleinma

i read also that the moon is moving away from the earth.. something like an inch a year.. or half inch..

would take some time to break free i guess

#### chroot

Staff Emeritus
Gold Member
Yep, the Earth is slowing its rotation, becoming tidally locked to the Moon. To conserve angular momentum, the Moon's orbit increases a little.

- Warren

#### kleinma

how come planets that are in the suns orbit don't drift towards the sun slowly?

#### chroot

Staff Emeritus
Gold Member
Originally posted by kleinma
how come planets that are in the suns orbit don't drift towards the sun slowly?
Er, why would they?

- Warren

#### kleinma

Originally posted by chroot
Er, why would they?

- Warren
gravity? i mean i obviously know that they don't drift towards the sun... but why not.. if the suns gravity pulls things towards it.. as earths gravity does, then why doesn't the sun draw the planets in?

#### subtillioN

Originally posted by kleinma
gravity? i mean i obviously know that they don't drift towards the sun... but why not.. if the suns gravity pulls things towards it.. as earths gravity does, then why doesn't the sun draw the planets in?
The gravitational pull of the sun is supposedly exactly offset by the tangential velocity of the earth. It is in a constant state of acceleration (falling) toward the sun but it's tangential velocity keeps it from ever reaching it. This newtonian assumption, of course, is how we "measure" the mass of these bodies, by simply fitting the equations to the observation of the orbit and the speed. But this in turn rests on further assumptions of the mass of the earth and the mass of the sun. The whole thing could be way off if a single one of these assumptions is incorrect, which the plasma model seems to suggest. The masses of these objects may be very much less than our current estimates.

The question is whether the gravitational newtonian interpretation is even correct, given the new Plasma models of the solar system. The regularity of the orbits (as seen in Bode's law and the same pattern is seen in the structure of the electron shells of an atom) may in fact be due to more of an electro-magnetic harmonic resonance than a simple gravitational accident. Such a purely gravitational scheme fails to explain the extreme regularity of the orbits of the solar system and much else including the rotational curve of the galaxies.

A good test would be to place an object in the orbit of the earth and cancel its tangential motion wrt the sun to see if it falls toward the sun. Does anyone know if this test has ever been done?

---------------

on a tangential note...

Does anyone know if the interplanetary probes are/were sent along the plane of the ecliptic of the solar system or if they are offset to escape the interstellar debris that collects upon this plane?

In all the images I have seen it seen they appear to be offset from the plane, but perhaps there have been a few that have traveled directly through another orbital intersection with the plane of the ecliptic?

There may be some data in those experiments which point to the resonance mechanism hinted at above.

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#### Janus

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Gold Member
Originally posted by subtillioN

A good test would be to place an object in the orbit of the earth and cancel its tangential motion wrt the sun to see if it falls toward the sun. Does anyone know if this test has ever been done?

Yes, everytime we've launched a probe into an orbit that takes it in closer to the Sun. In fact, if any of our "assumptions" of celestrial mechanics were of by any significant amount, none of our probes would have ever reached their destinations.

#### subtillioN

Originally posted by Janus
Yes, everytime we've launched a probe into an orbit that takes it in closer to the Sun. In fact, if any of our "assumptions" of celestrial mechanics were of by any significant amount, none of our probes would have ever reached their destinations.
Very true, but do you know if any of the probes have crossed the plane of the ecliptic at any of the planetary orbits?

#### Janus

Staff Emeritus
Gold Member
Originally posted by subtillioN
Very true, but do you know if any of the probes have crossed the plane of the ecliptic at any of the planetary orbits?
They all have. The probes are launched along Earth's orbital plane. They follow this trajectory until they reach that point where the Earth's and destination planet's orbital planes cross, at which point they do what is known as a "broken plane maneuver" , which tranfers them to the orbital plane of the destination planet.

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