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# Why do we always see the same side of the moon?

Fri 28 Mar, 2008 04:35 am
I understand that the moon's rotates at a rate equivalent to its revolution; meaning the same side of the moon always points toward the earth... but why?

Is it random cosmic placement? Is the moon directly rotating at the exact same rate, or are we seeing a new millimeter or inch of the moon every month? Or is it because one side of the moon has greater mass than the other causing it to stay pointed to the earth?

Have scientists examined this? What have they concluded?
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raprap

1
Fri 28 Mar, 2008 05:00 am
Tidal forces-- we can see the effect that the moon's gravity has on earth's oceans giving a high and low ebb on these liquids. Now recognize that Newton is correct and that every force is in equilibrium, those tidal forces are equally expressed by the earth on the moon. and the moon is smaller than the earth and even though the moon is not a liquid, those tidal forces express a tidal torque on moons mass that cause the rotation to match it's revolution.

A similar condition also exists between Mercury and the Sun---tidal forces force the rotation or Mercury to match the revolution.

Rap
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Vengoropatubus

1
Fri 28 Mar, 2008 06:00 pm
Does the surface of the Sun exhibit tidal effects that cause this tidal torque on the moon?
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curtis73

1
Sat 29 Mar, 2008 05:08 am
raprap wrote:
Tidal forces-- we can see the effect that the moon's gravity has on earth's oceans giving a high and low ebb on these liquids. Now recognize that Newton is correct and that every force is in equilibrium, those tidal forces are equally expressed by the earth on the moon. and the moon is smaller than the earth and even though the moon is not a liquid, those tidal forces express a tidal torque on moons mass that cause the rotation to match it's revolution.

A similar condition also exists between Mercury and the Sun---tidal forces force the rotation or Mercury to match the revolution.

Rap

While I agree that the earth's overall gravitational pull on the moon is equal to the moon's pull, your hypothesis suggests that the moon is unequally distributed in mass. Since the moon is a big chunk of rock, if it is being held in the same orientation to the earth, because of EARTH'S forces, that assumes that the moon is unbalanced.

I guess what I'm saying is; if the moon were of equal density and mass throughout its circumferential volume, the tides wouldn't change its motion. I think of it like those little marble balls in the Chinese fountains. They are of equally distributed mass, so when the upflow of water hits them, they spin uniformly. If they were unbalanced, they would either stay stationary or rock back and forth.

To further beat a dead horse... If the moon were of equal density all around, theoretically we could send a rocket up to start it spinning and nothing would change here on earth other than the side of the moon we see.

So... I'm wondering. Is the reason we see the same side because of the fact that the moon simply coincidentally is rotating at the same rate as its revolution, or is it because of an imbalanced mass causing it to be locked in to one position? The reason I ask is because; if its of equal mass distribution, the chances that its actually spinning at EXACTLY the same rate as its revolution, that is an insanely improbable mathematical anomoly. If its because of a density difference, its easy to explain.
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raprap

1
Sat 29 Mar, 2008 05:39 am
The moon isn't uniform, nor is it homogeneous, so the glass marble in a water stream really isn't a good analogy, even if you assume the stream flow is very laminar. The moon has a Geology (Selenology?) that is a factor of it's history just like that of the earth--and it has an elliptic bulge that formed as a result of the tidal forces generated in the earth moon system when the moon was still completely molten.

Hit the blue "Geology" in the first paragraph and you'll be magically transported to a wikipedia site on the geology of the moon, scroll to the section on Geologic (Seleneologic?) History and you'll see that the moon is not a static ball of rock, or dust--that it is subject to volcanism and techtonics.

Rap
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curtis73

1
Sat 29 Mar, 2008 06:25 am
Ok.. I'll check that link when its not 5am

So is it the imbalance of the moon that keeps its "heavy" side pointed toward us?
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Francis

1
Sat 29 Mar, 2008 06:33 am
Taking into account the moon's libration, it shows a bit more than really an half:

Quote:
At any time, only half of the Moon's surface is visible from Earth, but librations allow us to "peek around the edges." Over time, up to 59% can be observed, although near the edge, where the line of view is very slanted, not much detail can be made out. Nowadays artificial lunar satellites have mapped the Moon in great detail, so such extra coverage is no big deal. But before the space age, when astronomers were denied any view of the back of the Moon, any trick for increasing their coverage was appreciated.
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1
Sat 29 Mar, 2008 11:01 am
raprap wrote:
A similar condition also exists between Mercury and the Sun---tidal forces force the rotation or Mercury to match the revolution.

Nope.

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

Quote:
Until radar observations in 1965 proved otherwise, it was thought that Mercury was tidally locked with the Sun. Instead, it turned out that Mercury has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury's orbit makes this resonance stable. The original reason astronomers thought it was tidally locked was because whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, so showing the same face, which would be also the case if it were totally locked.

Quote:
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, one side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This synchronous rotation causes one hemisphere constantly to face the partner body. Usually, only the satellite becomes tidally locked around the larger planet, but if the difference in mass between the two bodies and their physical separation is small, both may become tidally locked to the other, as is the case between Pluto and Charon. This effect is employed to stabilize some artificial satellites.
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raprap

1
Sat 29 Mar, 2008 01:10 pm
Thanks DrewSad-- I try to learn something new every day--you've satisfied today.

Rap
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curtis73

1
Sun 30 Mar, 2008 08:57 am
Yummy answers. Thanks, folks.. they taste good
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g day

1
Sun 13 Apr, 2008 10:45 pm
I'd like to check something. When dinosaurs were alive I'd calculate the moon would have only been about 20,000 km away (based on a roughly constant recession rate of 3cm a year) - and tides would have been huge!

Are we saying the same face will always be looking at us - or that it changes slowly over time as the moon drifts away from us?
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spendius

1
Mon 14 Apr, 2008 04:53 am
At that distance how fast would it have been going relative to a point on the earth and how big would it have looked?

Wouldn't there have been whoosing tsunamis?
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g day

1
Mon 14 Apr, 2008 06:40 pm
Huge and fairly fast! The velocity would be about 13,555 km/hour for that separation (based on Average Earth radius + 20,000km plus Average Moon radius as the separation distance of the centres of mass).

Force = G * Mass Earth * Mass Moon/radius ^ 2 = Mass Moon * Velocity Moon ^2 / radius,

So Velocity Moon = (G * Mass Earth / radius) ^ 1/2

Angular size... arctan(Moon diameter in meters 3,476,000 / separation distance 20,000,000) = 9.86 degrees - so almost 20 times wider that it appears today!

So the Moon would orbit the Earth at this distance and angular velocity in just a minute or two over 13 hours!
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Setanta

1
Mon 14 Apr, 2008 07:12 pm
G__day, how do you know that the rate of divergence of the moon and the earth is a constant? If the moon had been "captured," but then began to pull away from the earth, the rate of divergence might be increasing, suggesting it were less in the past. I suspect, but don't know, that "giant" tides would have left some evidence.
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g day

1
Mon 14 Apr, 2008 11:52 pm
Setanta - I don't know - t'was just a postulate "if this then that"

Is it reasonable... yes with a very minor alteration - much larger tides would have slightly changed the rate of precession - but I intuit and stress slight. Said tides would have fallen with the square of distance and the total mass of all the worlds oceans is say 1.5 * 10 ^ 18 kgs (estimated as say average depth 10 km in a sphere of radius 7,000 kms), that is still only one part in 10,000 of the Earth's total weight.

Yes sky and to a certain extent the crust and upper mantle also come into effect - so calculating the exact rate of precession is more tricky - you need to know the average density and fluidity of the Earth post cataclysmic crash / capture scenario - to get precession rate correct.

Our best understanding is a capture scenario - where a body about Mars's size strikes the Earth with a bit more than a glancing blow - leaving a bit more than half its mass (the heavier core) to sink into the Earth and the other half to eject into space and eventually gravitationally coalesce into the Moon - several billion years ago.

Satellites slow down as they hit space dust and molecules in space - a tiny but consistent drag. But for a mass the size and volume of the Moon, I ponder you could almost ignore a density of 3 hydrogen molecules per cubic metre of space - spread over a billion years with impacts every second - the momentum is too high to simply show this drag!

I have a friend who's a radio astronomer - I'll ping him tomorrow!
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Kayyam

1
Mon 14 Apr, 2008 11:53 pm
Hold on there folks! We still haven't explained tidal locking yet. In what way do tidal forces create a torque? Tidal forces are the result of the gravitational gradient caused by the 'front' of the moon being nearer than its 'back'. In other words, tidal forces would encourage a sphere to develop a prolate bulge directed along the gravitational axis. Thus if the moon were completely rigid and not subject to even the slightest deformation, it would spin independently of its orbit and synchronization would be completely coincidental.

According to Wikipedia the moon does deform a little. But the actual bulging lags behind the gradient that induces it. Let us place a green dot on the surface of the moon nearest the Earth. If the moon is spinning faster than its orbit then the green dot appears to be moving forward (in the direction of its orbit/rotation). Its bulge too would be on the forward side. This creates a backwards "restoring" torque due to the asymmetry of mass - namely the fact that the front bulge is nearer than the rear one.

Likewise if the green dot appears to be moving backward (i.e. the moon is spinning too slowly), then the prolate bulge lags behind. The bulge and the dot like each other This causes a torque that increases the rotation speed.

How beautiful is that?

Now what does this say about the effect of rotation speed on locking? Does it affect the number of years that it takes to synchronize from an initially asynchronous orbit?

What happens when the orbit is not perfectly circular?

/Kayyam
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g day

1
Tue 15 Apr, 2008 12:04 am
Its extremely unusual for orbit to be circular - alah Kepler orbits are normally elliptical!

Too I am not bothering with equinox's every 27,000 years!

http://www.perceptions.couk.com/precess.html

Then gravitational & tidal processes - [a land-tide, estimated at about 8 inches (20 cms) is our present slow roller-coaster] - slowed down the Moon's SPIN until locked facing Earth, and that the Moon's ORBIT speed somehow also decreased significantly.

But continual transfer of Earth's spin-energy (angular momentum) to the Moon, through those same effects, now causes the Moon's ORBIT - circling the Earth - to increase. This drives the Moon out to greater orbit distances - at an estimated rate of 4 metres per century.

Here's some more workings-out at - Cornell.edu's Astronomy page

http://curious.astro.cornell.edu/question.php?number=124

The Moon's orbit (its circular path around the Earth) is indeed getting larger, at a rate of about 3.8 centimeters per year. (The Moon's orbit has a radius of 384,000 km.) I wouldn't say that the Moon is getting closer to the Sun, specifically, though--it is getting farther from the Earth, so, when it's in the part of its orbit closest to the Sun, it's closer, but when it's in the part of its orbit farthest from the Sun, it's farther away.

The reason for the increase is that the Moon raises tides on the Earth. Because the side of the Earth that faces the Moon is closer, it feels a stronger pull of gravity than the center of the Earth. Similarly, the part of the Earth facing away from the Moon feels less gravity than the center of the Earth. This effect stretches the Earth a bit, making it a little bit oblong. We call the parts that stick out "tidal bulges." The actual solid body of the Earth is distorted a few centimeters, but the most noticable effect is the tides raised on the ocean.

Now, all mass exerts a gravitational force, and the tidal bulges on the Earth exert a gravitational pull on the Moon. Because the Earth rotates faster (once every 24 hours) than the Moon orbits (once every 27.3 days) the bulge tries to "speed up" the Moon, and pull it ahead in its orbit. The Moon is also pulling back on the tidal bulge of the Earth, slowing the Earth's rotation. Tidal friction, caused by the movement of the tidal bulge around the Earth, takes energy out of the Earth and puts it into the Moon's orbit, making the Moon's orbit bigger (but, a bit pardoxically, the Moon actually moves slower!).

The Earth's rotation is slowing down because of this. One hundred years from now, the day will be 2 milliseconds longer than it is now.

This same process took place billions of years ago--but the Moon was slowed down by the tides raised on it by the Earth. That's why the Moon always keeps the same face pointed toward the Earth. Because the Earth is so much larger than the Moon, this process, called tidal locking, took place very quickly, in a few tens of millions of years.

Many physicists considered the effects of tides on the Earth-Moon system. However, George Howard Darwin (Charles Darwin's son) was the first person to work out, in a mathematical way, how the Moon's orbit would evolve due to tidal friction, in the late 19th century. He is usually credited with the invention of the modern theory of tidal evolution.

So that's where the idea came from, but how was it first measured? The answer is quite complicated, but I've tried to give the best answer I can, based on a little research into the history of the question.

There are three ways for us to actually measure the effects of tidal friction.

* Measure the change in the length of the lunar month over time.

This can be accomplished by examining the thickness of tidal deposits preserved in rocks, called tidal rhythmites, which can be billions of years old, although measurements only exist for rhythmites that are 900 million years old. As far as I can find (I am not a geologist!) these measurements have only been done since the early 90's.

* Measure the change in the distance between the Earth and the Moon.

This is accomplished in modern times by bouncing lasers off reflectors left on the surface of the Moon by the Apollo astronauts. Less accurate measurements were obtained in the early 70's.

* Measure the change in the rotational period of the Earth over time.

Nowadays, the rotation of the Earth is measured using the Very Long Baseline Interferometry, a technique using many radio telescopes a great distance apart. With VLBI, the positions of quasars (tiny, distant, radio-bright objects) can be measured very accuarately. Since the rotating Earth carries the antennas along, these measurements can tell us the rotation speed of the Earth very accurately.

However, the change in the Earth's rotational period was first measured using eclipses, of all things. Astronomers who studied the timing of eclipses over many centuries found that the Moon seemed to be accelerating in its orbit, but what was actually happening was the the Earth's rotation was slowing down. The effect was first noticed by Edmund Halley in 1695, and first measured by Richard Dunthorne in 1748--though neither one really understood what they were seeing. I think this is the earliest discovery of the effect.
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NickFun

1
Tue 15 Apr, 2008 12:30 am
Here's the dark side. Nuthin special about it. The side we see is nicer:

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g day

1
Tue 15 Apr, 2008 12:40 am
Curious I choose 20,000 kms, this is reasonable close to the Roche limit http://en.wikipedia.org/wiki/Roche_limit - the point at which the Earth gravitationally tears another smaller planetoid of similar density apart - so the Moon capture could have been done with a prolonged (days - weeks) very near miss, rather than a direct hit or glancing blow!

http://www.time.com/time/magazine/article/0,9171,830457,00.html
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gungasnake

1
Tue 15 Apr, 2008 03:48 am
Stranger question is the phase lock between Earth and Venus, i.e. the fact that Venus shows us the same face at inferior conjunctions. We're WAY too far from Venus for gravity to play any part in that.
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