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How does our Moon stabilize the Earth's spin vector?



 
 
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  #1  
Old August 14th 11, 10:31 AM posted to sci.astro.research
stargene
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Posts: 43
Default How does our Moon stabilize the Earth's spin vector?

Conundrum....
Any number of sites state that the presence of our moon over billions
of years has more or less stabilized the direction of the earth's spin
as it rotates around its own axis. The absence of such a moon would
have caused strong changes or wobbles in the angle of earth's spin
with reference to the ecliptic plane, and would have caused very
extreme
variations in insolation, seasons and climate. This would have been
detrimental to higher forms of life.

However, these same sites, some by authoritative astronomers do not
say precisely how the moon performs this stabilization. I am left
only
with my intuition and guesswork.

Does it have to do at all with earth's tidal interactions coupling
with the
moon's orbital motion?

Does it have to do with the fact that most of the earth-moon's angular
momentum resides in the moon?

thanks,
Gene
  #2  
Old August 14th 11, 12:30 PM posted to sci.astro.research
eric gisse
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Posts: 303
Default How does our Moon stabilize the Earth's spin vector?

stargene wrote in news:mt2.0-24975-1313314285
@hydra.herts.ac.uk:

Conundrum....
Any number of sites state that the presence of our moon over billions
of years has more or less stabilized the direction of the earth's spin
as it rotates around its own axis.


zuh?

The Earth's spin axis is stable, with or without the moon, because of
conservation of angular momentum. You can have some precession via rigid
(and slightly less rigid) body dynamics, but that isn't meaningful here.

Think carefully. What would make the spin axis unstable?

The absence of such a moon would
have caused strong changes or wobbles in the angle of earth's spin
with reference to the ecliptic plane, and would have caused very
extreme
variations in insolation, seasons and climate. This would have been
detrimental to higher forms of life.


That's why the spin axes of all the other major plantes (save Neptune)
are more-or-less perpendicular to the ecliptic? Because of moons?

Mercury, Venus, Mars, and all the gas giants have no moons or moons that
are a miniscule fraction of the planet's mass.

The argument has been made that the Moon was important to life forming
because the initial protoplanet got nailed by something ~Mars size,
stripping off a sizable chunk of the light elements that formed proto-
urf's crust. The end-result being that volcanism didn't get locked up,
like what we think happened to Venus.

Or it wasn't important at all, and is just a feature of the solar
system.


However, these same sites, some by authoritative astronomers do not
say precisely how the moon performs this stabilization. I am left
only
with my intuition and guesswork.

Does it have to do at all with earth's tidal interactions coupling
with the
moon's orbital motion?

Does it have to do with the fact that most of the earth-moon's angular
momentum resides in the moon?


The Earth and the Moon are tidally locked, which is what I suspect the
poorly-worded sites were referring to. This means the Earth rotates as
fast as the Moon orbits, which is why we always see the same side.

There's been a dampening of Earth's rotation by this process, but I
don't think that's what the sites are referring to.



thanks,
Gene

  #3  
Old August 14th 11, 02:12 PM posted to sci.astro.research
Phillip Helbig---undress to reply
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Posts: 629
Default How does our Moon stabilize the Earth's spin vector?

In article , eric gisse
writes:

stargene wrote in news:mt2.0-24975-1313314285
@hydra.herts.ac.uk:

Conundrum....
Any number of sites state that the presence of our moon over billions
of years has more or less stabilized the direction of the earth's spin
as it rotates around its own axis.


zuh?

The Earth's spin axis is stable, with or without the moon, because of
conservation of angular momentum.


That would be true if the Earth were isolated, but it is not. The
original poster is correct: the Moon keeps variations in the axial tilt
of the Earth smaller than they would otherwise be.

Think carefully. What would make the spin axis unstable?


Perturbations from other objects in the Solar System.

That's why the spin axes of all the other major plantes (save Neptune)
are more-or-less perpendicular to the ecliptic? Because of moons?


The giant planets are much more massive than the Earth.

Mercury, Venus, Mars, and all the gas giants have no moons or moons that
are a miniscule fraction of the planet's mass.


The giant planets are much more massive than the Earth.

There is evidence that the other terrestrial planets have undergone
large changes in their axial tilt.

While newsgroups have their uses, for basic questions, it might be
quicker to search the web. "Moon stabilizes Earth's axial tilt" gives
558,000 hits in Google. The first few I looked at look OK and
informative.
  #4  
Old August 17th 11, 09:12 AM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default How does our Moon stabilize the Earth's spin vector?

On Sun, 14 Aug 11 11:30:11 GMT, eric gisse wrote:
Think carefully. What would make the spin axis unstable?


You mean like if the planet's core spins at a different axis? Ever
heard of the magnetic pole? Depending on the plasma core, a planet
can certainly tumble.

That's why the spin axes of all the other major plantes (save Neptune)
are more-or-less perpendicular to the ecliptic? Because of moons?


So you think Uranus's tilt of 97deg to the ecliptic is perpendicular?
Eh wot?
  #5  
Old August 17th 11, 04:12 PM posted to sci.astro.research
Nicolaas Vroom[_2_]
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Posts: 16
Default How does our Moon stabilize the Earth's spin vector?

"stargene" schreef in bericht
...
Conundrum....
Any number of sites state that the presence of our moon over billions
of years has more or less stabilized the direction of the earth's spin
as it rotates around its own axis. The absence of such a moon would
have caused strong changes or wobbles in the angle of earth's spin
with reference to the ecliptic plane, and would have caused very
extreme
variations in insolation, seasons and climate. This would have been
detrimental to higher forms of life.

thanks,
Gene


My first reaction was that the Moon does not stabilize the Earth
(its precession) but has the reverse effect.
In general any two object system is 100% stable.
For Example the Sun and the Earth or the Earth and the Moon.

When you consider a third object this becomes less stable.
For Example the Sun Earth Moon system,
specific with the earth following an elliptic path.
For Example if you remove the Moon from such a system
the result again will be stable including the precession of
the Earth.

I did a search with Google: Stabilization Earth obliquity Moon
and I found the following five links:

1. http://www.nature.com/nature/journal.../361615a0.html
Lasker 1993: Stabilization of the Earth's obliquity by the moon
2. http://en.wikipedia.org/wiki/Future_...arth#Obliquity
Wikipedia: Future of the Earth
3. http://www.imcce.fr/Equipes/ASD/pers.../jxl_moon.html
Lasker 1997: Earth without a moon
4. http://en.wikipedia.org/wiki/Axial_t...iod_variations
Wikipedia: Axial tilt
5. http://io9.com/5829438/earth-doesnt-need-the-moon
Aug 2011 Earth does not need the moon.

It is important to remark that all(?) of those articles speak about
the future of our Solar system and not about the past
There is no mention what will happen if the sun all of a sudden
disappears. In fact this happens slowly.
What you need are two simulations (starting from the present):
1. One with the moon
2. One without the moon.
And observe when obliquity reaches 90 degrees.

Accordingly to link 5 the influence of the other planets
is the most important factor (Jupiter)
See Also link 4 for text related to Mars.

Nicolaas Vroom
http://users.pandora.be/nicvroom/
  #6  
Old August 17th 11, 04:13 PM posted to sci.astro.research
eric gisse
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Posts: 303
Default How does our Moon stabilize the Earth's spin vector?

Eric Flesch wrote in news:mt2.0-22392-1313568759
@hydra.herts.ac.uk:

On Sun, 14 Aug 11 11:30:11 GMT, eric gisse wrote:
Think carefully. What would make the spin axis unstable?


You mean like if the planet's core spins at a different axis? Ever
heard of the magnetic pole? Depending on the plasma core, a planet
can certainly tumble.


I stand by what I said, but I apparently vastly underestimated the
perturbative effects of the other plants. Until very recently (hehe) I
never knew that the other plants (save the obvious one) had any major
changes in their spin axes.

I do know that the rotational pole and magnetic pole tend to be in
different spots, but I'm unsure if that's exactly relevant.

The reason is the actual location of the magnetic pole does not, in my
understanding, correspond to something majorly moving but rather the
resultant of the Earth's dynamo's various multipole moments changing.
The magnetic north pole is more along the lines of a sum of forces
pointing in a particular direction rather than an actual object.


That's why the spin axes of all the other major plantes (save Neptune)
are more-or-less perpendicular to the ecliptic? Because of moons?


So you think Uranus's tilt of 97deg to the ecliptic is perpendicular?
Eh wot?


That was one of the things I probably should have looked up. Replace
Neptune with Uranus. I knew one of them got whacked with something big
and had its' spin axis put in an "interesting" direction, I just guessed
wrong :P
  #7  
Old August 18th 11, 08:36 AM posted to sci.astro.research
Eric Flesch
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Posts: 321
Default How does our Moon stabilize the Earth's spin vector?

On Wed, 17 Aug 11, eric gisse wrote:
The magnetic north pole is more along the lines of a sum of forces
pointing in a particular direction rather than an actual object.


By that standard the Sun would not flip its magnetic field, but it
does, every two solar cyles. Geologic evidence shows the Earth flips
its field also. It's become clear that Earth has a core which turns
freely as it will -- it's so dense that it is to the mantle as the
mantle is to the atmosphere. It follows immediately that the magnetic
field originates from there. I suspect your "sum of forces" is more
accurately cast as the "sum of our knowledge".

I wrote:
So you think Uranus's tilt of 97deg to the ecliptic is perpendicular?


That was one of the things I probably should have looked up. Replace
Neptune with Uranus. I knew one of them got whacked with something big
and had its' spin axis put in an "interesting" direction, I just guessed
wrong :P


Then there's also Venus, which rotates retrograde. Did you think it
slowed from its original rotation, stopped, and decided to go the
other way? Well here are three clues:

(1) Its axial tilt is termed as 177 degrees.
(2) Its siderial rotational period is close to its year (hint hint --
as though it were once locked to its orbit)
(3) Venus has NO MOON.
  #8  
Old August 19th 11, 07:14 PM posted to sci.astro.research
Nicolaas Vroom[_2_]
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Posts: 16
Default How does our Moon stabilize the Earth's spin vector?

"eric gisse" schreef in bericht
...

I stand by what I said, but I apparently vastly underestimated the
perturbative effects of the other plants.


The same picture arises when you study the perihelion shift of Mercury
caused by the other planets:
Venus 276,8 arc sec
Earth 89 arc sec
Mars 2.2 arc sec
Jupiter 155,2 arc sec
Saturnus 7.5 arc sec
Together roughly 531 arc sec per century

Without those planets Mercury would be more stable.

As I already said "you" should perform a simulation with and without
the Moon in order to answer the question what the influence is.
Without the Moon for example there are no tides.
To take that into account is a real challange.

Nicolaas Vroom
  #9  
Old August 19th 11, 09:01 PM posted to sci.astro.research
Hans Aberg[_3_]
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Posts: 18
Default How does our Moon stabilize the Earth's spin vector?

On 2011/08/19 20:14, Nicolaas Vroom wrote:

As I already said "you" should perform a simulation with and without
the Moon in order to answer the question what the influence is.
Without the Moon for example there are no tides.
To take that into account is a real challange.


Yes, there would, due to the sun; according this link, about half the size:
http://www.astronomycafe.net/qadir/q106.html

Hans
 




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