Unlocking a tide locked body

On 7 okt, 00:34, Erik Max Francis wrote:
DJensen wrote:
On Oct 5, 6:21 am, Erik Max Francis wrote:
DJensen wrote:
I did a quick search through Google Groups but couldn't find a thread
where this had been discussed.
What would it take to unlock a tide locked planet or moon, even for a
brief period of a few hundred thousand or million years? A precise
bombardment seems like an obvious solution, but would it really work?
Well, unlock it in what way? Apply enough angular momentum to mean that
the body experiences differential rotation relative to its revolution,
no matter how slight? Or do you mean _major_ tidal unlocking, so that
it is dramatically spun up or spun down?

Using Earth for comparison, how slight is slight? A 240 hour day?
2,400? More?

Earth isn't tide-locked, so using it for comparison doesn't really mean
much. The Moon would be a better example, because it _is_ tide-locked,
and its rotation/revolution period is 28 days. By "slight" I'm talking
changes to, say, 27.5 or 28.5 days. Things that would result in no
tidal lock (at least temporarily; it would re-lock very rapidly over
astronomical timescales), but not a huge change in the rotation speed.
It would still require an enormous amount of angular momentum.

Suppose that the Moon were somehow turned 180 degrees in longitude,
and stopped there. What would its period of free libration then be?
Still 1056 days? Shorter than 1056 days? Longer than 1056 days but
still real? Or imaginary?

On Oct 7, 12:13*pm, Crown-Horned Snorkack
wrote:

Suppose that the Moon were somehow turned 180 degrees in longitude,
and stopped there...

Then it would still be tidally locked. Stopped is stopped.
Furthermore, turning the Moon 180° still aligns the long axis with the
Earth-Moon line... so it's still locked in that sense as well. Even if
you turned it just 90°, you wouldn't necessarily have a change in
rotational state (although you might have some significant Moon-quakes
- i'n not sure what the relaxation timescale is under those
conditions).

What would its period of free libration then be? Still 1056 days?

Hmm. What liberation are you refering to? I was thinking about the
liberation due to the eccentricity of the lunar orbit. There are
diurnal liberations of the Moon (due to viewing angle from the limbs
of the Earth), monthly liberations of the Moon (due to the orbital
eccentricity and therefore varying orbital speed), and even longer
term components (due to the precession of the lunar orbit with respect
to the Earth's equatorial bulge). But none of these are due to a
variation in the rotation - they are due to variations in orbital
elements. The rotation is very constant.

--
Brian Davis

On 2008-10-08, Brian Davis wrote:

Hmm. What liberation are you refering to? I was thinking about the
liberation due to the eccentricity of the lunar orbit. There are
diurnal liberations of the Moon (due to viewing angle from the limbs
of the Earth), monthly liberations of the Moon (due to the orbital

*Gnnnh*... must... resist... the urge... argh, okay, enough!

Ahem. "Libration". Ell-eye-bee-are-ay-tee-eye-oh-en. No "e".

http://dictionary.reference.com/browse/libration

Sorry.

--
Ilmari Karonen
To reply by e-mail, please replace ".invalid" with ".net" in address.
(Of course, the laws of Usenet karma dictate that this post shall
inevitably contain at least one egregious and embarrassing misspelling.)

On Oct 8, 8:59*am, Ilmari Karonen wrote:

...Sorry.

Groan. No your not. And yes, I deserved that

--
Brian Davis

Brian Davis ) writes:
On Oct 8, 8:59=A0am, Ilmari Karonen wrote:

...Sorry.

Groan. No your not. And yes, I deserved that

Luna is a very lib(e)rated lady.

--John Park

On 8 okt, 15:01, Brian Davis wrote:
On Oct 7, 12:13*pm, Crown-Horned Snorkack
wrote:

Suppose that the Moon were somehow turned 180 degrees in longitude,
and stopped there...

Then it would still be tidally locked. Stopped is stopped.

If the Moon were turned through 1 degrees of longitude and stopped
there, it would not remain stopped.

Furthermore, turning the Moon 180° still aligns the long axis with the
Earth-Moon line... so it's still locked in that sense as well. Even if
you turned it just 90°, you wouldn't necessarily have a change in
rotational state (although you might have some significant Moon-quakes
- i'n not sure what the relaxation timescale is under those
conditions).

What would its period of free libration then be? Still 1056 days?

Hmm. What liberation are you refering to?

Free libration, as I said.

I was thinking about the
liberation due to the eccentricity of the lunar orbit. There are
diurnal liberations of the Moon (due to viewing angle from the limbs
of the Earth), monthly liberations of the Moon (due to the orbital
eccentricity and therefore varying orbital speed),

Which are forced librations.

and even longer
term components (due to the precession of the lunar orbit with respect
to the Earth's equatorial bulge). But none of these are due to a
variation in the rotation - they are due to variations in orbital
elements. The rotation is very constant.

Very constant. But it is constant precisely because, should a small
change occur, the rotation would then change.

OK, there were a few things I was missing (including noticing the word
"free". Sorry.).

I'm not sure what the free libration period would be if you magicly
displaced the Moon by 1°, or 10°. Or how long it would take to damp
out - that's an important question from the standpoint of pumping up
the free libration to turn it into a free rotation (a libration of
greater than +/- 90°). I wonder how much of the current free libration
(slight as it is) is due to some past impact(s). After all, w have a
good record of all recent impacts on the Moon (photos), as well as
their size (crater size) and some idea of their age. It would seem the
thing that would be hard to reconstruct is the impact angle (not
preserved in the crater shape except for extremely glancing impacts).

--
Brian Davis

"Brian Davis" wrote in message
...
OK, there were a few things I was missing (including noticing the word
"free". Sorry.).

I'm not sure what the free libration period would be if you magicly
displaced the Moon by 1°, or 10°. Or how long it would take to damp
out - that's an important question from the standpoint of pumping up
the free libration to turn it into a free rotation (a libration of
greater than +/- 90°). I wonder how much of the current free libration
(slight as it is) is due to some past impact(s). After all, w have a
good record of all recent impacts on the Moon (photos), as well as
their size (crater size) and some idea of their age. It would seem the
thing that would be hard to reconstruct is the impact angle (not
preserved in the crater shape except for extremely glancing impacts).

--
Brian Davis

==============================================
You are way off base, old son.
Imagine if you can the Moon having a highly eccentric orbit, very long
in the major axis and very short in the minor axis.
http://www.androcles01.pwp.blueyonde...rbit/Orbit.xls
With the same face toward you at apogee and perigee, and with the
Moon rotating at a constant rate, one rotation per revolution, what face
do you see when it is crossing the minor axis?

On Oct 8, 4:42*pm, "Androcles" wrote:

You are way off base, old son.

If you mean "Brian was off base, and Crown-horned Snorkack was right
on", you're correct. There's a big difference between the forced and
the free libration of the Moon, and yes, the free libration is due to
inconstant rotation of the Moon.

Imagine if you can the Moon having a highly eccentric orbit, very long
in the major axis and very short in the minor axis... what face do you
see when it is crossing the minor axis?

That's the forced libration. This sub-thread is about the free
libration (and, occasionally, my bad spelling).

--
Brian Davis

"Brian Davis" wrote in message
...
On Oct 8, 4:42 pm, "Androcles" wrote:

You are way off base, old son.

If you mean "Brian was off base, and Crown-horned Snorkack was right
on", you're correct. There's a big difference between the forced and
the free libration of the Moon, and yes, the free libration is due to
inconstant rotation of the Moon.

Imagine if you can the Moon having a highly eccentric orbit, very long
in the major axis and very short in the minor axis... what face do you
see when it is crossing the minor axis?

That's the forced libration. This sub-thread is about the free
libration (and, occasionally, my bad spelling).

--
Brian Davis


Ah, you must be writing from
rec.arts.sf.science, a science fiction and arts newsgroup.

http://antwrp.gsfc.nasa.gov/apod/ap070902.html
http://en.wikipedia.org/wiki/Libration

Modelled:
http://www.androcles01.pwp.blueyonde.../Libration.htm

There is no "forced libration", any change in the Moon's orbit is
precession and will not be noticeable for thousands of years.
Libration is noticeable in a month.

Please don't cross-post your nonsense to sci.astro, which is a
science newsgroup.