Hi,
Perhaps a silly question here, but here it goes anyhow.

The moon rotates around the earth equally as it rotates around it's axis, so we only will see one and the same side of the moon.(The dark side being the side away from earth).

So the question really is. What is the origin/trigger that causes the Moon and Earth to influence eachother in such way? As far as I know of all moons we observed on the solar system this characteristic only appeared between Earth and our Moon.

Patrick

What dark side?

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Well, first "dark side of the moon" is misleading. The side that faces away from the Earth is called the far side of the moon - the dark side is simply whichever side faces away from the sun (That changes over time. It's why we get full moons, new moons, gibbous moons, etc.)

The reason that one side of the moon always faces Earth is because it is tidally locked with us. A good explanation can be found here.

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Thanks for the link. I am a bit confused about this 'Tidal locking'. Is this something that always occurs? E.g. Earth is not tidally locked with the Sun.

And this phenomenon is common - in fact ubiquitous - among moons that are reasonably close to their planets. The moons of Mars, Galilean and inner moons of Jupiter, and moons of Saturn out to at least Iapetus follow this so-called synchronous or tidally-locked rotation. Pluto and Charon have gone so far as to be tidally locked to each other.

(Iapetus does have a dark side, but nothing to do with solar illumination!)

And in fact, the process is still occurring between the Earth and Moon. We are slowly becoming locked with the Moon.

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Charon and Pluto are locked. (NO comment on whether Pluto is a planet please!)

Mercury is locked to the Sun, actually a 3:2 spin-orbit resonance which accounts for it's large libration.

More to the point, all other known moons in the Solar System are gravitationally locked to the parent planet except Phoebe.

Venus is locked to the Earth so that at closest approach it always has the same side facing us.

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I'm not too sure about this one... Venus exibits phases like the moon does because we can see it move toward the far side of the sun, but I doubt we always see the same part of the planet. It all just looks the same because all we can see is the top of the atmosphere.

Of course, if you can show me a site that proves me wrong, I'll be more than willing to fix myself a crow sandwich

Not only is the orbit (year) of Venus in a 5:8 resonance with the Earth year, but its rotation (day) is in a 3:2 resonance with the Earth year!

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The same side of Venus does not always face us. But when Venus is at closest approach to Earth, we always see the same side.

http://en.wikipedia.org/wiki/Tidal_locking

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Actually small irregular satellites (at least whose rotational periods are known) are not gravitationally locked to their parent planets.

Is it possible that this resonance is not real but actually just a coincidence? I mean, is there really any mechanism that could cause such resonance? I find it hard to believe that Earth could have that much influence on the rotation of Venus.

DoubleJ,

I said "Venus is locked to the Earth so that at closest approach it always has the same side facing us." That doesn't mean the same side always faces us. We can easily determine what side faces us with radar.

See
http://www.naic.edu/~nolan/radar/planets.html

Kullat,

Yes, there is a mass ratio between moon and primary below which tidal locking does not occur. I don't know what it is. At some point you have to stop calling the little stuff moons. How about when they stop tidally locking *self serving suggestion*?

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Kullat,

I have a hard time in believing in such coincidences when tidal effects and resonances are so common.

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It's not a silly question! But what I would suggest you do is a google search on some of the other moons in the solar system. You can find data tables that list the orbital and rotational periods of moons.

For example, if you do a google search for : moon orbital rotational period jupiter


You'll get this site as the first hit:
http://nssdc.gsfc.nasa.gov/planetary...ansatfact.html


Scroll down to the section titled: Orbital parameters. All of the moons that have S as the rotation period, that means that the orbital and rotational periods are the Same. So that would be the answer to your question:

From the link I gave you, Io, Europa, Ganymede, Callisto, Metis, Adrastea, and Amalthea are all gravitationally locked, just like our moon. If you could stand on the surface of Jupiter, you would see the same face of all those moons all the time. That's a lot of moons. Isn't it? And that's just Jupiter.

So, this isn't a wierd coincidence at all. It's the natural end state.

I should have done a google search at first, indeed.(sorry)

One more question, if 'tidal locking' is common, why isn't Earth for example than tidally locked with the Sun? I assume distance has to do with it.

I meant possible resonances between Earth and Venus only. Of course resonances are very typical between planets and their moons.

It does. Tidal forces are inversely proportional to the CUBE of distance - which is why solar tides on Earth are smaller than lunar tides. Moon exerts more tidal force on Earth than Sun does - and Moon has not succeeded in locking Earth rotation yet!

Indeed. Like Jupiter's large moons, all the large moons of Saturn up to Iapetus are tidally locked -- excluding Hyperion. Actually it is unusual because it is not tidally locked. It does not rotate like normal body at all, but "tumbles" chaotically as it orbits Saturn.

My guess is that the rotational relationship is a coincidence, but the orbital relationship is resonant.

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T. Anderson

You know, it's possible to generate electricity from tides. You can, for example, put a turbine in a tube and when the tide comes in, the rising water will push air out of the tube and drive the turbine.

There is this principle of science called maximum entropy. Eventually, everything settles down. Eventually, all the H2 in the universe will be used up. All the stars will go out. Every rock that's on top of a hill ready to roll down will do so. etc. etc. There is no possible way to generate power forever. Eventually, it will all be used up. But, planets and moons will still orbit each other. As long as you didn't take energy from them (like with a teather) they will keep right on orbiting forever* pretty much.

So what I'm getting at is, if bodies didn't become tidallly locked there would be an end-run around entropy. You could keep getting free electricity from tides forever. So the answer to the question, "is this something that always occurs" is a very definite yes.

* As it stands, the universe will "only" let you get energy this way for billions and billions of years. (shakes fist) damn you nature, you win again!

Although on some questions a Google search could lead you astray....especiallywith some of the woo-woos out there in cyberspace. Asking questions of knowledgeble people is never wrong.

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441!!!! :)

Anyone else ever wonder if the clarifiers of "dark side" aren't being a wee bit too literal? Heck, I've been one myself, but lately I've had second thoughts.

I grew up when much of Africa had recently been called "darkest" and it wasn't because people thought that the sun didn't shine there. It was because it was (relatively) unknown and hidden, almost unexplored by outsiders.

Amongst almost all the mulitple dictionary definitions of dark I see is one meaning of "secret, hidden, mysterious". The Merriam-Webster etymology of dark has it as: akin to Old High German tarchannen to hide, so the "illuminated" meaning is probably the more recent, century-wise.

I would defiantly say the far side of the moon is definitely the dark side, in that particular sense of the word. It is hidden -- from us non-astronauts.

(Yeah, I've seen the pictures, but I couldn't really describe it or even pick it out of a photo lineup of cratered bodies.)

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I've seen the dark side of the Moon: when the Earth shines onto the dark side, it can illuminate it quite nicely;

this phenomenon is called Earthshine,
or, quaintly, the 'New Moon in the Old Moon's arms'.

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I would have said no. The time it will take the Earth to be tidally locked to the Sun is longer than the projected lifetime of the Earth, so it will not occur.

It depends upon just how much energy you take out of the system. If the rotation is relatively frictionless, then it will not occur or occur too slowly to matter. Another factor is the strength of the shape of the planet--an example is Mercury, which is not tidally locked to the Sun. The reason for that appears to be that at closest approach to the Sun, the gravitational effect on a non-tidal bulge gives Mercury a "boost" which counteracts the rest of the tidal slowing.

A Thousand Pardons,

Mercury is considered tidally locked to the sun, just not at 1 to 1 but at a 3-2 spin orbit resonance which is a stable situation caused by tidal effects and maintained by tidal effects.


A very good explanation of tides and resonances here.

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I don't think so. We discussed this last fall, and Eroica found an online PDF that seemed to explain it. It seems to depend upon a permanent deformation of Mercury--without which, Mercury would probably continue on to a true tidal lock.
They mention that Mercury actually speeds up to the point where the Sun reverses itself in the sky, near perihelion.

Mercury speeds up at periapsis because of it's significant eccentricity of 0.2056. As per Kepler the orbit sweeps equal area in equal time. 0.2 is a high eccentricity. Mercury minimum orbital velocity is ~39 kms and max is ~59kms. This eccentricity is much more responsible for the spin orbit resonance because of the variation in tides from periapsis to apoapsis, a factor of 5.

Every where I have looked considers Mercury to be locked tidally, just not 1 to 1.

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Check out Eroica's link.

A most interesting paper. I have read it. It is an excellent example of how to obtain a PHD without qualitative evidence.

Firstly, I quote:

They then go on to say:

They mention "permanent deformations"

But give no reference or evidence for these deformations.


And then:

Nothing is perfectly rigid, hence the tidal effects.

They give no evidence or references for the assertion that Mercury is non-uniform. They do however go on to say that:

This is not exactly convincing, but I might accept my laurels based on such a paper if I were able to pull it off. It is very well couched, it includes just the right amount of mathematics and is just vague enough to escape criticism for making definitive statements not based on actual science. It's not a scam, but it is highly speculative and does not have a lot of evidence to back it up. It contains enough waffle words to escape any censure.

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