More than tides affecting the moons distance?

Discussing with some friends about how the moon is shifting away from
the earth at something like 2 inches per year, I was asked how much of
this could be attributed to Jupiters gravity effects. I'm thinking it
would be safe to say that when the moon is between the Earth and
Jupiter, it would indeed be pulled toward Jupiter some small amount,
but correspondingly 15 days later the earth would be between the moon
and Jupiter, Jupiter's gravity would now be pulling the moon back
toward the Earth. And so Jupiter's effect can be considered
cancelled. Am I making sense? Can anyone shed more thoughts? Stan

wrote in message
...
: Discussing with some friends about how the moon is shifting away from
: the earth at something like 2 inches per year, I was asked how much of
: this could be attributed to Jupiters gravity effects. I'm thinking it
: would be safe to say that when the moon is between the Earth and
: Jupiter, it would indeed be pulled toward Jupiter some small amount,
: but correspondingly 15 days later the earth would be between the moon
: and Jupiter, Jupiter's gravity would now be pulling the moon back
: toward the Earth. And so Jupiter's effect can be considered
: cancelled. Am I making sense? Can anyone shed more thoughts? Stan


Forget Jupiter for now, the questions you need to ask yourself a
Why does the Moon keep one face toward the Earth?
Why doesN'T the Earth keep one face toward the Moon?

The answer is "tides".
The far side of the Moon has further to go in its orbit
than the near side. If we split the Moon into two halves
then the far side would take more than a month and the near
side less than a month to complete an orbit. With the two
halves coupled that cannot happen, but there is a force acting
which means the Moon keeps one face toward the Earth.
Eventually the same force will cause the Earth to keep one
face toward the Moon as we lose our angular momentum
which is heating the Earth internally and to a lesser extent
the ocean floor as the water slides over it twice a day
producing friction.
Now... there are two centres of rotation for the Earth,
one the Earth's own axis which passes through the poles
and the other is the common barycentre of the Earth-Moon
system, which is also below the surface. When the Earth
slows so that it faces the Moon, there will be only one
centre of rotation. It isn't so much the Moon is moving away
from the Earth as it is the Earth is moving away from the
Earth-Moon barycentre as the tides gradually decrease.

As to your Jupiter argument, Jupiter is a lot further away
and a whole lot smaller than the Sun, the orbit of the Earth-
Moon system is elliptical, so if there is any third body involved
it will be the Sun first.

Dear pstanle...:

On Sep 20, 10:32 am, wrote:
Discussing with some friends about how the moon
is shifting away from the earth at something like 2
inches per year,

.... ~3.72 cm / year, a little less than 1½ in/year ...

I was asked how much of this could be attributed to
Jupiters gravity effects. I'm thinking it would be safe
to say that when the moon is between the Earth and
Jupiter, it would indeed be pulled toward Jupiter
some small amount, but correspondingly 15 days
later the earth would be between the moon and
Jupiter, Jupiter's gravity would now be pulling the
moon back toward the Earth. And so Jupiter's effect
can be considered cancelled. Am I making sense?

Yes, but it is action oriented along the orbital path that "boosts".
So it is when Jupiter is located "ahead" or "behind" that it affects
lunar recession. You might get a small boost from Jupiter's
rotation... rather than its orbital motion.

Can anyone shed more thoughts? Stan

Predictions of the "position" of the Moon at any given time relies on
up to five different objects, exclusive of Earth. Jupiter is only one
of them.

David A. Smith

In sci.astro message , Thu,
20 Sep 2007 14:32:00, posted:
Discussing with some friends about how the moon is shifting away from
the earth at something like 2 inches per year, I was asked how much of
this could be attributed to Jupiters gravity effects. I'm thinking it
would be safe to say that when the moon is between the Earth and
Jupiter, it would indeed be pulled toward Jupiter some small amount,
but correspondingly 15 days later the earth would be between the moon
and Jupiter, Jupiter's gravity would now be pulling the moon back
toward the Earth. And so Jupiter's effect can be considered
cancelled. Am I making sense? Can anyone shed more thoughts? Stan

The Sun is five times nearer and many times heavier; it would have a
much greater effect than Jupiter.

The actual cause is a combination of Lunar tides on Earth and the
Earth's rotation; energy is transferred to the Moon, I presume because
the rotation pulls the tidal bulges ahead of the Earth-Moon line so that
the Moon thereby gets a forwards component of force. It will be in
Wikipedia, I expect - http://en.wikipedia.org/wiki/Tidal_acceleration.

--
(c) John Stockton, Surrey, 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.

Thanks all. I've read Wiki, and think I understand enough about the
moon/tides relationship. It was the influences of other planets or
even the sun on the moon orbit that we were discussing.

If we know just consder the influence of the sun on the moon's orbit,
and for academic purposes "pretend" there are no other planets nor any
tidal forces that cause orbital changes.

Further, make other simplifying assumptions such as the mass of the
objects are constant and even, the moon and earth's orbits are
circular.....

Over a period of a months or years, when the moon has made a number of
complete orbits of the earth, would we end up with the moon in a
different orbit about the Earth? How would this orbit be different?

thanks for any inputs, Stan

wrote in message
...
: Thanks all. I've read Wiki, and think I understand enough about the
: moon/tides relationship. It was the influences of other planets or
: even the sun on the moon orbit that we were discussing.
:
: If we know just consder the influence of the sun on the moon's orbit,
: and for academic purposes "pretend" there are no other planets nor any
: tidal forces that cause orbital changes.
:
: Further, make other simplifying assumptions such as the mass of the
: objects are constant and even, the moon and earth's orbits are
: circular.....
:
: Over a period of a months or years, when the moon has made a number of
: complete orbits of the earth, would we end up with the moon in a
: different orbit about the Earth?

The Moon doesn't orbit the Earth, it orbits a common barycentre.
Think of the see-saw you played on as a child.
Heavy child nearer the fulcrum. Light child further from
the fulcrum.

O--^--------o

Now make the see-saw longer:
O--------^--------------------------------o
It still balances.
Now make the see-saw shorter:
O-^----o
It still balances.

: How would this orbit be different?
It will have a longer seesaw as the Earth loses rotation.
Angular momentum is conserved.

On Fri, 21 Sep 2007 13:46:15 GMT, "Androcles"
wrote:

The Moon doesn't orbit the Earth, it orbits a common barycentre.

Now make the see-saw longer:
O--------^--------------------------------o
It still balances.
Now make the see-saw shorter:
O-^----o
It still balances.

: How would this orbit be different?
It will have a longer seesaw as the Earth loses rotation.
Angular momentum is conserved.

I understand your point about the barycentre, although I've never had
anyone suggest that using the term "orbit" for the moon around the
earth was incorrect.

So I'm still wondering then, due to the sun's gravity as detailled in
my previous post, over a period of a months or years, when the moon
has made a number of orbits around a barycentre, would we end up with
the moon/earth orbitting a different barycentre?

wrote in message
...
: On Fri, 21 Sep 2007 13:46:15 GMT, "Androcles"
: wrote:
:
: The Moon doesn't orbit the Earth, it orbits a common barycentre.
:
: Now make the see-saw longer:
: O--------^--------------------------------o
: It still balances.
: Now make the see-saw shorter:
: O-^----o
: It still balances.
:
: : How would this orbit be different?
: It will have a longer seesaw as the Earth loses rotation.
: Angular momentum is conserved.
:
: I understand your point about the barycentre, although I've never had
: anyone suggest that using the term "orbit" for the moon around the
: earth was incorrect.

Even your terms "earth" and "moon" are incorrect. The noun "earth"
refers to the stuff you grow potatoes in, the pronoun "Earth" is a planet,
mostly ocean. Earth has one moon (noun), the Moon (or Luna) (pronoun).
In everyday parlance saying the Moon orbits the Earth is an acceptable
approximation to what actually happens but in a discussion such as
this it helps if we are a tad more precise.
In reality the Moon orbits the Sun more so than it does the Earth.

: So I'm still wondering then, due to the sun's gravity as detailled in
: my previous post, over a period of a months or years, when the moon
: has made a number of orbits around a barycentre, would we end up with
: the moon/earth orbitting a different barycentre?

There is only one barycentre for each pair of bodies. Three bodies,
three barycentres. The Sun-Moon barycentre will be deeper inside
the Sun than the Sun-Earth barycentre, the Earth-Moon barycentre
is nowhere near the Sun but below the surface of the Earth.
In an elliptical orbit both bodies are varying their distance from the
barycentre. You can actually see the Moon change its apparent size
in this animation, and also see more than 180 degrees of its surface.
http://antwrp.gsfc.nasa.gov/apod/ima...nation_ajc.gif
When objects change their apparent size they are moving toward
or away from the observer.
This simulation doesn't include tides but will give you some
insight into three-body systems.
http://faculty.ifmo.ru/butikov/Projects/Collection.html
We simply cannot say "if it were a circle" and predict the outcome,
three body systems are chaotic. If you want to know more about
chaos start he
http://www.imho.com/grae/chaos/chaos.html

When I say "cannot predict" I mean just that. We use a constellation
of 27 artificial satellites and millions of receivers on the ground
to give us a Global Positioning System (GPS) which depends
on knowing exactly where each satellite is, but the satellites are
perturbed in their orbits by the Moon and the Sun and we do not
know where they will be in ten years time. We've overcome the
problem by using fixed ground stations that upload position to each
satellite on a regular basis.
Trying to figure out where the Moon will be in 10,000 years time
is an impossible task, but even if we could it is not provable.
"God does not play dice" -- Einstein.
"Yes, I do. Einstein was a bad loser" -- God.

In sci.astro message ,
Fri, 21 Sep 2007 13:46:15, Androcles posted:

The Moon doesn't orbit the Earth, it orbits a common barycentre.


The Moon actually orbits the Sun, somewhat perturbed by the Earth. The
Moon's path is everywhere concave towards the Sun.

In a system with three bodies all of non-negligible mass, none of the
three bodies is in a simple orbit about any of the four barycentres.

--
(c) John Stockton, Surrey, 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.

"Dr J R Stockton" wrote in message
nvalid...
: In sci.astro message ,
: Fri, 21 Sep 2007 13:46:15, Androcles posted:
:
: The Moon doesn't orbit the Earth, it orbits a common barycentre.
:
:
: The Moon actually orbits the Sun,

No, the Moon orbits a barycentre it shares with the Sun.