Why is the rotation period of the moon exactly the same as the translation?
Ramon F Herrera
I know very little about astronomy, other than having an engineering
background. I have been watching programs such as "The Moon" in the
History Channel where they speculate on the different theories about
the moon's origin.
What amazes me is that such an obviously crucial fact is not even
mentioned. The two periods being so identical has to imply something.
Are there other bodies which exhibit this dancing rhythm?
They say that the moon is getting farther and farther from us,
correct? How does that affect the rotation? I am guessing that we will
finally start to see the dark side of the moon, albeit veeeeery
slowly. From what side is that going to happen?
-Ramon
Sam Wormley
Here's and engineering term-The moon's rotation is phase-locked
to the Earth and will remain that way as it walks away at 3.8 cm
per year (current rate). Many moons are phase-locked to their
parent bodies in the solar system.
The current theory of the origin of the moon... is the one best
supported by the evidence... of rock composition, age and measured
dynamics.
Ramon F Herrera
Thanks, Sam...
I hear that there are two types of satellites: those which were born
from the parent and those which were grabbed as they passed by.
Perhaps that refers to celetial bodies in general and not to moon-
planet systems in particular?
Is it just me or there is a numerical justification for the legitimate
satellites being (tending to be) phase-locked and the adoptive ones
not being?
I saw in one of the "Universe" programs that one of the planets has an
axis of rotation which is *perpendicular* to the normal? Boy, is the
universe weird.
-Ramon
Eugene Griessel
Uranus. It's axis of rotation is almost perpendicular to its orbit.
Probably the result of a collision with a large body at some stage in
it's life.
Eugene L Griessel
(A)bort, (R)etry, (T)ake down entire network?
- I usually post only from Sci.Military.Naval -
Sam Wormley
Because of density variation... rotating bodies will tend to
"phase-lock".... Some day that will be the case with the earth
also... rotating once per lunar orbit.
>
> I saw in one of the "Universe" programs that one of the planets has an
> axis of rotation which is *perpendicular* to the normal? Boy, is the
> universe weird.
For sure... and infinitely interesting!
>
> -Ramon
>
Guy Macon
Sam Wormley wrote:
> Here's and engineering term-The moon's rotation is phase-locked
> to the Earth and will remain that way as it walks away at 3.8 cm
> per year (current rate). Many moons are phase-locked to their
> parent bodies in the solar system.
...
> Because of density variation... rotating bodies will tend to
> "phase-lock".... Some day that will be the case with the earth
> also... rotating once per lunar orbit.
I am not sure whether the density variation has anything to
do with the actual phase locking, other than determining
which side will face the parent when the lock is complete.
To go from a non-phase-locked state to a phase-locked state,
something has to transfer rotational energy to/from the moon.
Otherwise the law of conservation of angular momentum will
preclude any change in rotational speed. That energy transfer
comes from tidal-pull-induced movement/friction. When the lock
is achieved, the tidal-pull-induced movement/friction starts
reducing the back-and-forth wobble. When that is done, the
moon no longer experiences tides and thus stays locked.
--
Guy Macon <http://www.guymacon.com/> Guy Macon <http://www.guymacon.com/>
Guy Macon <http://www.guymacon.com/> Guy Macon <http://www.guymacon.com/>
Guy Macon <http://www.guymacon.com/> Guy Macon <http://www.guymacon.com/>
Guy Macon <http://www.guymacon.com/> Guy Macon <http://www.guymacon.com/>
Helpful person
At last the correct explanation!
William Hamblen
<ra...@conexus.net> wrote:
>What amazes me is that such an obviously crucial fact is not even
>mentioned. The two periods being so identical has to imply something.
>Are there other bodies which exhibit this dancing rhythm?
Where we have the data, the periods of revolution of most natural
satellites are the same as their periods of revolution around the
planet.
The reason is that owing to tidal forces on the satellite from the
gravity of the planet the period of rotation of the satellite will
come to equal the period of its revolution around the planet. Isaac
Newton wrote out the reason why in Principia Mathematica back in the
17th Century.
Bud
lal_truckee
>
> Is it just me or there is a numerical justification for the legitimate
> satellites being (tending to be) phase-locked and the adoptive ones
> not being?
Numerically, it's age.
Gotta give the host sufficient time to act on the tides.
Quadibloc
> What amazes me is that such an obviously crucial fact is not even
> mentioned. The two periods being so identical has to imply something.
> Are there other bodies which exhibit this dancing rhythm?
For the Moon to turn one side always to the Earth is not a
coincidence. It is a natural result of the effect of the Earth's
gravity on the Moon. The Moon has a nonzero size, so while something
orbiting the Earth at the distance of the Moon's center would have the
Moon's orbital period, something at the distance of the closest part
of the Moon to us would orbit more quickly.
The Earth exerts tidal forces on the Moon, just as the Moon does on
the Earth. This slows down the rotation of both bodies, until each
permanently faces the other, and so the Moon now always faces the
Earth, and, eventually, the Earth will always face the Moon.
John Savard
Curtis Croulet
> finally start to see the dark side of the moon, albeit veeeeery
> slowly. From what side is that going to happen?
There is no "dark side of the Moon," if by that phrase you mean a side that
is dark all the time. The *far* side of the Moon, which receives light
during its rotation just as does the near side, has been viewed by the
astronauts and it has been imaged.
--
Curtis Croulet
Temecula, California
33°27'59"N, 117°05'53"W
Al
|the Earth. This slows down the rotation of both bodies, until each
|permanently faces the other, and so the Moon now always faces the
|Earth, and, eventually, the Earth will always face the Moon.
Not technically correct... Earth (and Moon) will be "consumed" by the
sun millenia before Earth's rotation locksteps with Moon. A niggling
point but for the record...
Al
Helpful person
> There is no "dark side of the Moon," if by that phrase you mean a side that
> Temecula, California
> 33°27'59"N, 117°05'53"W
Yes there is. Many people believe that it is Pink Floyd's best
album. Personally I prefer Wish You were Here.
Quadibloc
> On Dec 27, 8:33 pm, "Curtis Croulet"
>
> > There is no "dark side of the Moon," if by that phrase you mean a side that
> > is dark all the time.
>
> album.
You misphrased that. You should have said:
That's true, but if he had meant a Pink Floyd album instead, there
would have been one.
since, clearly, it _is_ true there is no side of the Moon that is dark
all the time.
Of course, if electromagnetic radiation in the 400-700nm region
reflects off the surface of an inhabited planet's natural satellite,
and it points away from that planet so that nobody sees it, does it
give any light?
If a tree falls in the forest, and there is no one to hear it, does
the tree really collapse into a "fallen" eigenstate, or will the ratio
between the upright and fallen eigenstates in a superposition of them
merely change gradually?
Is light a form of electromagnetic radiation, and sound a longitudinal
pressure wave, or is it only light and sound if it evokes qualia in
the minds of conscious observers?
John Savard
Helpful person
> If a tree falls in the forest, and there is no one to hear it, does
> the tree really collapse into a "fallen" eigenstate, or will the ratio
> between the upright and fallen eigenstates in a superposition of them
> merely change gradually?
>
More to the point:
If a man speeks an opinion in the middle of the forest where his wife
can't hear him is he still wrong?
IAP
> |The Earth exerts tidal forces on the Moon, just as the Moon does on
> |the Earth. This slows down the rotation of both bodies, until each
> |permanently faces the other,
The moon's rotation must have been opposite to the earth's before the
lockstep.
Why the moon began it's life rotating opposite to the earth's rotation is
mysterious
unless the earth was once surrounded with a ring of material that orbited
opposite to it's rotation
before coalescing and becoming the moon.
Quadibloc
> The moon's rotation must have been opposite to the earth's before the
> lockstep.
Why?
John Savard
Sam Wormley
>
> The moon's rotation must have been opposite to the earth's before the
> lockstep.
Evidence please.
Chris L Peterson
>The moon's rotation must have been opposite to the earth's before the
>lockstep.
Not at all. Any pair of orbiting bodies will eventually become tidally
locked, regardless of the relative directions of rotation. The fact that
the Moon is tidally locked to the Earth tells us something about its
initial rotation rate, but nothing at all about direction.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
Andrew Smallshaw
>
> For the Moon to turn one side always to the Earth is not a
> coincidence. It is a natural result of the effect of the Earth's
> gravity on the Moon. The Moon has a nonzero size, so while something
> orbiting the Earth at the distance of the Moon's center would have the
> Moon's orbital period, something at the distance of the closest part
> of the Moon to us would orbit more quickly.
>
> The Earth exerts tidal forces on the Moon, just as the Moon does on
> the Earth. This slows down the rotation of both bodies, until each
> permanently faces the other, and so the Moon now always faces the
> Earth, and, eventually, the Earth will always face the Moon.
I've read this elsewhere before, but is it actually true? I'll
admit that I haven't considered the maths in the necessary amount
of detail: I'll leave that to someone who actually understands the
subject. Ignore the sun for the time being and consider the
Earth-Moon system in isolation.
The Earth is gradually slowing down as a result of the tidal forces,
according to figures I have just found this is happening at a rate
of 1 second every 50000 years. During this time the radius of the
Moon's orbit will increase by 2.5 km.
Now I know it isn't valid to scale these values indefinitely because
the underlying equations change as the Earth slows and the Moon's
orbit increases, but consider this values over a period of time
enough to treble the length of the Earth's day. At this time the
Moon's orbital radius will have increased by 439,000 km, i.e. more
than doubled. Long before this point is reached it seems seems
intuitive that the increased Earth-Moon distance would have reduced
the strength of tidal effects dramatically and everything would
have changed much less than simply projecting today's figures into
the future.
Although that simply extends the timescales it would appear to me
that the system would tend towards (though never actually reach)
a stable state where the moon is in a greatly enlarged orbit, tidal
effects are negligible and the Earth still has a surplus of angular
momemtum over being tidally locked.
Does anyone with a better grasp of the necessary equations have
any input on this theory?
--
Andrew Smallshaw
and...@sdf.lonestar.org
IAP
>>The moon's rotation must have been opposite to the earth's before the
>>lockstep.
>
> the Moon is tidally locked to the Earth tells us something about its
> initial rotation rate, but nothing at all about direction.
Ok, so my idea or opinion is as vaild as yours.
Chris L Peterson
>Ok, so my idea or opinion is as vaild as yours.
It's not an opinion. The physics of tidal locking are well understood.
Do a search on "tidal locking" and you'll find the formula used to
derive the tidal lock time. The formula uses only the magnitude of the
initial rotation rate of the satellite, and doesn't use the rotation
rate (or direction) of the parent body at all.
IAP
"Chris L Peterson" <c...@alumni.caltech.edu> wrote in message
news:610bn3hh3m9ejh5l0...@4ax.com...
> On Fri, 28 Dec 2007 22:24:02 GMT, "IAP" <I...@nospam.net> wrote:
What about relativistic frame dragging effects?
Chris L Peterson
>What about relativistic frame dragging effects?
They're trivially small compared to ordinary tidal effects, unless
you're talking about something getting tidally locked to a black hole.
Even then, I expect that tidal forces would destroy the satellite before
it could get close enough to the BH for frame dragging to be
significant.
IAP
>> The moon's rotation must have been opposite to the earth's before the
>> lockstep.
>
> Evidence please.
Not possible..wasn't there at the time
Quadibloc
> Although that simply extends the timescales it would appear to me
> that the system would tend towards (though never actually reach)
> a stable state where the moon is in a greatly enlarged orbit, tidal
> effects are negligible and the Earth still has a surplus of angular
> momemtum over being tidally locked.
Someone has noted that the Earth is not expected to become tidally
locked to the Moon prior to the Sun leaving the Main Sequence and
making the issue moot.
I suspect, though, that if this were not a consideration, the fact
that the Moon recedes from the Earth, reducing tidal forces, would not
lead to an asymptotic solution where the Earth is still rotating more
quickly than the Moon orbits it, even after the Moon has receded to an
infinite distance.
This is because we are dealing with a system with conservation of
energy - the Moon will only recede from Earth in proportion to the
energy transferred to it tidally from slowing the Earth's rotation,
and as long as tidal forces are non-zero, some transfer of energy will
take place. Thus, a solution where the difference between the angular
velocity of the Earth's rotation and that of the Moon's orbit is
asymptotically greater than or equal to some finite non-zero value is
not possible, because energy transfer would be taking place at a rate
that is always above some finite non-zero minimum.
What about a solution where this discrepancy tends asymptotically
towards zero, but remains non-zero at any finite time? That might
indeed be the case, but then after a finite time, the situation would
be indistinguishable for practical purposes from a mutual tidal lock.
John Savard
IAP
Sorry, wasn't there at the time.
Guy Macon
Al wrote:
...assuming, of course, that we don't attain the ability to control
the sun's fusion by then. :)
inv...@example.com
IAP wrote:
First, please don't make asserions if you are unwilling to
provide evidence backing them up.
Second, you don't have to "be there" to have evidence that
something happened. I wasn't alive during the Civil War,
but I have strong evidence that it happened.
Third, there must be *some* reason why you asserted that the
moon's rotation must have been opposite to the earth's before
the lockstep. Where did you get that idea? Did you read it
somewhere? Did you calculate it? Make it up to make yourself
appear smart? Did the voices in your head tell you to write that?
What were you thinking?