Re: Orbital precession w/o GR


"Eric Gisse" <jowr.pi@xxxxxxxxx> escreveu na mensagem
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On Dec 3, 11:24 am, "JMA" <NOS...@xxxxxxxxx> wrote:
"bz" <bz+...@xxxxxxxxxxxxxxxxxxxx> escreveu na
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"JMA" <NOS...@xxxxxxxxx> wrote
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My [not so clear] point was that they mean [to paraphrase slightly]
is
that "the precession of mercury is the slow shift of the point of
nearest approach to the sun along the orbital plane," whatever the
cause of said drift.

After taking into account the gravitational effects of all the
planets,
there was still some 'precession' left over. They first attributed it
to a small [as yet to be observed] planet inside mercury's orbit, but
that theory was rejected because such an orbit would not be stable.

GR came to the rescue and explained the drift.

So far I'm trying to figure out the problem based on Newton.

Then look at how precession of the equinoxes works.

I've looked and now I can tell you that orbit precession is
like a rigid body motion. The earth looks like a spinning top.

It isn't.

Damn Gisse comes again.
Above I wanted to say that Earth precession is like a rigid
body problem. Instead of "earth" I've said "orbit".


Fluid motion from inside the planet along with the non-rigid material
properties of the crust make the Earth an elastic-but-mostly-rigid
body.

Yep, make it with a 26,000 years period please ?

Precession of the equinoxes has to do with the non-sphericality of the
Earth. Specifically, the Earth's quadrupole moment and its'
interaction with the Moon.

Right, but the quadrupole moment means peanuts.
Why you talking about peanuts?



But, orbits are not rigid body problems.
Please see below.



When you say, "the precession of mercury is the slow shift of the
point of nearest approach to the sun along the orbital plane" how
does that point shift.
1 - The point becomes closer, or far away, from the sun within
the plane of the orbit.

NOT what I am talking about. Not to say that this might not happen, but
this is NOT precession.

2 - The distance to the Sun is always the same, but the point
moves in a plane orthogonal of the orbit plane.
(The distance to the Sun is always the same since rotation occurs
along a line that includes the Sun and is orthogonal to the line
defined by the perihelion-sun-aphelion.)

The oval 'pictures' of the 'orbit' do not exactly overlap [with respect
to
'the fixed stars' and a heliocentric frame of reference]

Keep in mind the FACT that the rotation of the SUN around its axis and
the revolution of the sun around the galactic center and the drag from
each passing planet has an effect on that ellipse.

I guess you are right here.

No specific constant 'torque' need be supplied, but the planet
'Vulcan'
would have supplied such a torque if Vulcan existed.

Torque needs to be orthogonal to the plane of the orbit.

That is your understanding.

'Precession' can be used as a model by assuming a rigid body rotating
and
calculating the [averaged] impulsive torques that each planet exerts on
every other body in the solar system.

No, I've been studying the problem and I've realised that
the model cannot be that of a rigid body
Please see more below.

That's because the planet isn't rigid.

What planet ?
I'm talking about orbits, where a point mass rotates in
elliptical motion.
Can't you see what the subject is ?



That is, the vector torque is in the plane of the orbit.

The model is a rotating rigid body. It has angular momentum.
Periodic small taps provide something which can be modeled as a
constant 'torque'.

No, I cannot agree.
If one chose a set of (x,y,z) axis for a coordinate system,
so that the orbit plane is the plane (x,y) and the rotation
occurs along the z axis. Precession occurs in the (x,y) plane,
and the z axis never changes position (like if it was fixed in space).
The angular momentum vector is colinear with the z axis
too, so that angular momentum vector never changes
during the precession motion.

Why are you confusing rigid body motion and orbital motion when you
JUST SAID they are not the same?

You are the one still confusing both.
Wake up and try to understand the subject.


The angular momentum vector is constant in both direction and
magnitude in orbital motion under a central force. The same can NOT be
said for rigid body motion.

I agree.




This is NOT rigid body motion.

Gyroscopic precession occurs in a plane orthogonal
to the plane (x,y) of the orbit, so that angular momentum
vector changes direction as a reaction to an external
applied torque.
Gyroscopic motion means that angular momentum
cross product with a torque creates precession.

Where on Earth do you get this stuff?

Well, forget this one.

Precession is NOT angular momentum crossed with torque.

The precession angular vector points where angular momentum
crossed with torque points. That was what I had in mind.


People call "gyroscopic precession" and "orbital precession"
the same term "precession", but these two motions are
in fact orthogonal to each other.

Stop confusing orbital motion and rigid body motion.

That was exactly my point.
Orbital motion is not rigid body motion.


The terms describe the SAME DAMN THING - the angular movement of a
fixed point on a specific plane as time goes on.

Now you turn around 180 degrees again.
Make up your mind.


In the case of "orbit precession" the angular momentum
vector never changes direction (is fixed in space).

No. Study some classical mechanics and general relativity.

Haaa.....??
So, now you saying that orbital precession is a rigid
body problem.

I'm going to tell you that I don't even want to read the
rest of your crap, since you don't make any sense.
(Read the links on my previous post and come back
again when you understand the problem).


.

Relevant Pages

  • Re: Orbital precession w/o GR
    ... nearest approach to the sun along the orbital plane," whatever the ... there was still some 'precession' left over. ... like a rigid body motion. ... Precession of Earth's perihelion is not like a rigid body problem. ...
    (sci.physics.relativity)
  • Re: Orbital precession w/o GR
    ... nearest approach to the sun along the orbital plane," whatever the ... there was still some 'precession' left over. ... The model is a rotating rigid body. ...
    (sci.physics.relativity)
  • Re: Orbital precession w/o GR
    ... nearest approach to the sun along the orbital plane," whatever the ... there was still some 'precession' left over. ... that theory was rejected because such an orbit would not be stable. ... Fluid motion from inside the planet along with the non-rigid material ...
    (sci.physics.relativity)
  • Re: Orbital precession w/o GR
    ... nearest approach to the sun along the orbital plane," whatever the ... there was still some 'precession' left over. ... that theory was rejected because such an orbit would not be stable. ... orbits are not rigid body problems. ...
    (sci.physics.relativity)
  • Re: Orbital precession w/o GR
    ... precession of mercury orbit, ... The rotational motion of the axis of a spinning body, ... it is you that needs to study rigid body motion. ... I believe kinetic energy comes from the kinetic energy fairy. ...
    (sci.physics.relativity)