Question about the "coincidence" of the "dark side of the moon"

[Jay R. Yablon]

For a change of pace on this Super Bowl Sunday, I have a question
totally unrelated to anything else I have talked about here.

We all know that the moon revolves about the earth and at the same time
rotates at virtually identical rates, so that one never sees the dark
side of the moon.

A few questions:

1) Has it been empirically determined to how many decimal places is
this so? If we are to come back 100,000 years from now, would today's
dark side still be out of view, or would there have been some
incremental change so that a fraction of what is now hidden from view
would rotate into view? One has to think that to some decimal place,
this is not totally exact.

2) From a standpoint of planetary evolution, based on physical
reasoning, what does this remarkable coincidence between the moon's
revolution about the earth and its rotation about its axis teach us, if
anything, about the origin and evolution of the earth-moon system?

3) Are their other planet / moon systems that we know of which have
this same remarkable coincidence between lunar revolution and rotation?

[[Mod. note -- This might better be asked over in sci.astro.research,
but...
1. The dark side would still be out of view, even billions of years
into the future, i.e. the Earth/Moon system is "tidally locked".
2. It's consistent with the modern scenario for the Moon's formation,
in which a Mars-sized planetesmial hit the proto-Earth very early
after the solar system formed, and blasted a lot of debris out into
space. That temporarily formed a ring, and later condensed into
the moon. [This nicely explains why the moon is so utter lacking
in volatiles -- they all boiled off during the impact/ring time.]
The moon thus formed *much* closer to the earth than it is now,
so the tidal damping of its spin was much faster, giving ample
time for its spin to become tidally locked to its orbit. Over
the 4+ billion years since its formation, the moon has gradually
receded from the earth (this is also driven by tidal forces),
and its spin has stayed locked (i.e. it has slowed as the moon's
orbital period has slowed).
3. Pluto/Charon is the other one that comes to mind right away.
The period there is around 6.4 days. Sun/Mercury is a 3:2
spin-orbit resonance, i.e. Mercury's spin period is 2/3 of
its orbital period.

See, for example, chapter 10 of
J. Kelly Beatty, Carolyn Collins Petersen, Andrew Chaikin, Eds,
"The New Solar System", 4th Edition
Cambridge U.P., March 1999,
430 pages 504 line diagrams 124 colour plates 32 tables 64 graphs
Paperback ISBN 0-521-64587-5
http://books.cambridge.org/0521645875.htm
for more discussion on the moon's origin.
-- jt]]

I also point out that when there is a solar eclipse, the moon appears to
be virtually the same size as the sun, which means that the ratio:

diameter(sun)/diameter(moon)
=distance(earth-sun)/distance(earth-moon)

I am curious how precise this is, and whether this also rather
remarkable coincidence tells us anything, based on physical reasoning,
about the origins and evolution of the solar system?

[[Mod. note -- It's precise to within 0.5% or so -- you can work this
out for yourself by observing that when we have an annular solar eclipse,
that's just the apparent diameter of the moon being close to its minimum
due to the eclipse happening close to the perigee of the moon's orbit
about the earth.

So far as I know, it's "just" a coincidence, and doesn't tell us anything
at all "deep" about the origin or evolution of the solar system.
-- jt]]

Thanks,

Jay.
____________________________
Jay R. Yablon
Email: jyablon@nycap.rr.com
co-moderator: sci.physics.foundations
Weblog: http://jayryablon.wordpress.com/

[Dirk Bruere at NeoPax]

Jay R. Yablon wrote:
> For a change of pace on this Super Bowl Sunday, I have a question
> totally unrelated to anything else I have talked about here.
>
> We all know that the moon revolves about the earth and at the same time
> rotates at virtually identical rates, so that one never sees the dark
> side of the moon.

Just a brief note - the "dark side" of the moon is no darker than the
side facing us. It's misnamed. The correct terminology is "far side".

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.onetribe.me.uk/wordpress/?cat=5 - Our podcasts on weird stuff

[Ian Parker]

The Earth raised tides on the Moon. This caused the Moon's rotation to
slow down and eventually stop relatrive to its orbit.

There are many other instances.

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

Much more interesting is where rotation and orbital motion are in
multiples.

http://www.phy.ohiou.edu/~astro/DispatchColumns/07-18-2006.pdf

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

The presence of resonance is particularly interesting. It indicates
that a complete lock may not be the most stable state. There are
implications in other fields of Physics.

- Ian Parker

[Phillip Helbig---remove CLOTHES to reply]

In article <6ummpvFg760dU1@mid.individual.net>, "Jay R. Yablon"
<jyablon@nycap.rr.com> writes:

> For a change of pace on this Super Bowl Sunday, I have a question
> totally unrelated to anything else I have talked about here.
>
> We all know that the moon revolves about the earth and at the same time
> rotates at virtually identical rates, so that one never sees the dark
> side of the moon.

First: There is no dark side of the Moon really. As a matter of fact,
it's all dark. The previous two sentences come at the end of Pink
Floyd's famous DARK SIDE OF THE Moon album. You should be saying the
FAR side of the Moon. At full Moon, the dark side and the far side are
the same. At new Moon, there is no overlap. Otherwise, intermediate.
(Actually, the entire Moon is quite dark, even when lit. It has a very
low albedo, about the same as black asphalt. It only looks bright
compared to the dark sky surrounding it.)

> A few questions:
>
> 1) Has it been empirically determined to how many decimal places is
> this so? If we are to come back 100,000 years from now, would today's
> dark side still be out of view, or would there have been some
> incremental change so that a fraction of what is now hidden from view
> would rotate into view? One has to think that to some decimal place,
> this is not totally exact.

First, it's not exact even at the present, since the orbit is not
circular. The rotation rate is approximately constant, but the rotation
rate depends on the distance. Thus, the Moon "wobbles" a bit. Due to
this wobble, and the fact that its orbit is inclined to the ecliptic, we
can see about 59% of the Moon from the Earth (though obviously not all
at the same time). Second, this situation is not a coincidence, but is
due to tidal locking. As such, it will stay that way. With time, the
Moon recedes from the earth and the rotation rate slows down, but in
step. (This means the Moon was closer in the past, and will be farther
away in the future. Thus, it IS a coincidence that now, for a
relatively short time in the history of the Earth, it has the same
angular size as the Sun, making a total solar eclipse possible.)

> 2) From a standpoint of planetary evolution, based on physical
> reasoning, what does this remarkable coincidence between the moon's
> revolution about the earth and its rotation about its axis teach us, if
> anything, about the origin and evolution of the earth-moon system?

Nothing. This state of affairs would have evolved whatever the origin
of the moon, given enough time.

> 3) Are their other planet / moon systems that we know of which have
> this same remarkable coincidence between lunar revolution and rotation?

Yes, many and, as mentioned above, it's not even remarkable.

> I also point out that when there is a solar eclipse, the moon appears to
> be virtually the same size as the sun, which means that the ratio:
>
> diameter(sun)/diameter(moon)
> =distance(earth-sun)/distance(earth-moon)
>
> I am curious how precise this is, and whether this also rather
> remarkable coincidence tells us anything, based on physical reasoning,
> about the origins and evolution of the solar system?
>
> [[Mod. note -- It's precise to within 0.5% or so -- you can work this
> out for yourself by observing that when we have an annular solar eclipse,
> that's just the apparent diameter of the moon being close to its minimum
> due to the eclipse happening close to the perigee of the moon's orbit
> about the earth.

The minimum apparent diameter is at apogee, not perigee.

> So far as I know, it's "just" a coincidence, and doesn't tell us anything
> at all "deep" about the origin or evolution of the solar system.
> -- jt]]

Since the distance of the Moon from the Earth varies, so does the ratio
of the angular diameters. But they are rather close---for now (see
above).

What is unusual is the size of the Moon compared to the size of the
Earth. No other moon/planet combination comes close. (Charon/Pluto
would, but Pluto is no longer considered to be a planet.)

[Jay R. Yablon]

"Dirk Bruere at NeoPax" <dirk.bruere@gmail.com> wrote in message
news:6umuqqFg1h5rU1@mid.individual.net...
> Jay R. Yablon wrote:
.. . .
>
> Just a brief note - the "dark side" of the moon is no darker than the
> side facing us. It's misnamed. The correct terminology is "far side".

Of course that is so, but as Phillip notes elsewhere in this thread,
that is from the famous Pink Floyd album, and I could not resist taking
that bit of artistic license.

Jay.

[[Mod. note -- Quoted signature snipped. -- jt]]

[Dirk Bruere at NeoPax]

Jay R. Yablon wrote:
> "Dirk Bruere at NeoPax" <dirk.bruere@gmail.com> wrote in message
> news:6umuqqFg1h5rU1@mid.individual.net...
>> Jay R. Yablon wrote:
> . . .
>> Just a brief note - the "dark side" of the moon is no darker than the
>> side facing us. It's misnamed. The correct terminology is "far side".
>
> Of course that is so, but as Phillip notes elsewhere in this thread,
> that is from the famous Pink Floyd album, and I could not resist taking
> that bit of artistic license.

Next time I come across a member of Pink Floyd I shall have to make a
formal complaint about the inaccurate terminology. Better late than
never eh?

Meanwhile, to stay on topic, is the tidal force responsible for slowing
the rotation of the moon a function of the speed of rotation of the
Earth or Moon? In other words, is the tidal lock "final" or is it still
continuing at an ever decreasing rate?

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.onetribe.me.uk/wordpress/?cat=5 - Our podcasts on weird stuff

[Phillip Helbig---remove CLOTHES to reply]

In article <6urfndFgq4luU1@mid.individual.net>, Dirk Bruere at NeoPax
<dirk.bruere@gmail.com> writes:

> >> Just a brief note - the "dark side" of the moon is no darker than the
> >> side facing us. It's misnamed. The correct terminology is "far side".
> >
> > Of course that is so, but as Phillip notes elsewhere in this thread,
> > that is from the famous Pink Floyd album, and I could not resist taking
> > that bit of artistic license.
>
> Next time I come across a member of Pink Floyd I shall have to make a
> formal complaint about the inaccurate terminology. Better late than
> never eh?

As I also noted, they also "correct" this at the very end of the album.

Did you notice that Stephen Hawking provides some, err, background
vocals on "Keep Talking" on THE DIVISION BELL? One last thought on rock
music and astronomy (and "better late than never"): Not long ago, Brian
May was finally awarded his doctoral degree.

> Meanwhile, to stay on topic, is the tidal force responsible for slowing
> the rotation of the moon a function of the speed of rotation of the
> Earth or Moon? In other words, is the tidal lock "final" or is it still
> continuing at an ever decreasing rate?

The rate of rotation of the Earth is slowing down. To conserve angular
momentum, the Moon is also moving away from the Earth, which lengthens
its orbital period. The rate of rotation of the Moon slows down to
match the longer orbital period.

[Dr J R Stockton]

In sci.physics.research message <6ummpvFg760dU1@mid.individual.net>,
Sun, 1 Feb 2009 19:01:07, Jay R. Yablon <jyablon@nycap.rr.com> posted:
>
>1) Has it been empirically determined to how many decimal places is
>this so? If we are to come back 100,000 years from now, would today's
>dark side still be out of view, or would there have been some
rear
>incremental change so that a fraction of what is now hidden from view
>would rotate into view? One has to think that to some decimal place,
>this is not totally exact.

It is in stable lock, small deviations being restored by tidal forces
and the oscillation dampened by tidal dissipation.

At least most libration is essentially geometric; in part because the
Moon's axial rotation is essentially uniform, whereas its orbital motion
is not; and in part because the relevant axes are not all parallel.

BUT : once those are accounted for, is there any detectable residual
oscillation, and if so of what person would it have?

AND : does the lock always remain as the Moon recedes?

--
(c) John Stockton, nr London, UK. ?@merlyn.demon.co.uk Turnpike v6.05 MIME.
Web <URL:http://www.merlyn.demon.co.uk/> - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

[Ian Parker]

The Earth raised tides on the Moon. This caused the Moon's rotation to
slow down and eventually stop relatrive to its orbit.

There are many other instances.

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

Much more interesting is where rotation and orbital motion are in
multiples.

http://www.phy.ohiou.edu/~astro/DispatchColumns/07-18-2006.pdf

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

The presence of resonance is particularly interesting. It indicates
that a complete lock may not be the most stable state. There are
implications in other fields of Physics.

- Ian Parker

[Phillip Helbig---remove CLOTHES to reply]

In article <6ummpvFg760dU1@mid.individual.net>, "Jay R. Yablon"
<jyablon@nycap.rr.com> writes:

> For a change of pace on this Super Bowl Sunday, I have a question
> totally unrelated to anything else I have talked about here.
>
> We all know that the moon revolves about the earth and at the same time
> rotates at virtually identical rates, so that one never sees the dark
> side of the moon.

First: There is no dark side of the Moon really. As a matter of fact,
it's all dark. The previous two sentences come at the end of Pink
Floyd's famous DARK SIDE OF THE Moon album. You should be saying the
FAR side of the Moon. At full Moon, the dark side and the far side are
the same. At new Moon, there is no overlap. Otherwise, intermediate.
(Actually, the entire Moon is quite dark, even when lit. It has a very
low albedo, about the same as black asphalt. It only looks bright
compared to the dark sky surrounding it.)

> A few questions:
>
> 1) Has it been empirically determined to how many decimal places is
> this so? If we are to come back 100,000 years from now, would today's
> dark side still be out of view, or would there have been some
> incremental change so that a fraction of what is now hidden from view
> would rotate into view? One has to think that to some decimal place,
> this is not totally exact.

First, it's not exact even at the present, since the orbit is not
circular. The rotation rate is approximately constant, but the rotation
rate depends on the distance. Thus, the Moon "wobbles" a bit. Due to
this wobble, and the fact that its orbit is inclined to the ecliptic, we
can see about 59% of the Moon from the Earth (though obviously not all
at the same time). Second, this situation is not a coincidence, but is
due to tidal locking. As such, it will stay that way. With time, the
Moon recedes from the earth and the rotation rate slows down, but in
step. (This means the Moon was closer in the past, and will be farther
away in the future. Thus, it IS a coincidence that now, for a
relatively short time in the history of the Earth, it has the same
angular size as the Sun, making a total solar eclipse possible.)

> 2) From a standpoint of planetary evolution, based on physical
> reasoning, what does this remarkable coincidence between the moon's
> revolution about the earth and its rotation about its axis teach us, if
> anything, about the origin and evolution of the earth-moon system?

Nothing. This state of affairs would have evolved whatever the origin
of the moon, given enough time.

> 3) Are their other planet / moon systems that we know of which have
> this same remarkable coincidence between lunar revolution and rotation?

Yes, many and, as mentioned above, it's not even remarkable.

> I also point out that when there is a solar eclipse, the moon appears to
> be virtually the same size as the sun, which means that the ratio:
>
> diameter(sun)/diameter(moon)
> =distance(earth-sun)/distance(earth-moon)
>
> I am curious how precise this is, and whether this also rather
> remarkable coincidence tells us anything, based on physical reasoning,
> about the origins and evolution of the solar system?
>
> [[Mod. note -- It's precise to within 0.5% or so -- you can work this
> out for yourself by observing that when we have an annular solar eclipse,
> that's just the apparent diameter of the moon being close to its minimum
> due to the eclipse happening close to the perigee of the moon's orbit
> about the earth.

The minimum apparent diameter is at apogee, not perigee.

> So far as I know, it's "just" a coincidence, and doesn't tell us anything
> at all "deep" about the origin or evolution of the solar system.
> -- jt]]

Since the distance of the Moon from the Earth varies, so does the ratio
of the angular diameters. But they are rather close---for now (see
above).

What is unusual is the size of the Moon compared to the size of the
Earth. No other moon/planet combination comes close. (Charon/Pluto
would, but Pluto is no longer considered to be a planet.)

[Dirk Bruere at NeoPax]

Jay R. Yablon wrote:
> For a change of pace on this Super Bowl Sunday, I have a question
> totally unrelated to anything else I have talked about here.
>
> We all know that the moon revolves about the earth and at the same time
> rotates at virtually identical rates, so that one never sees the dark
> side of the moon.

Just a brief note - the "dark side" of the moon is no darker than the
side facing us. It's misnamed. The correct terminology is "far side".

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.onetribe.me.uk/wordpress/?cat=5 - Our podcasts on weird stuff