PDA

View Full Version : precession of moon's orbit

grav
2006-Sep-12, 01:51 AM
The precession of the moon's orbit is one revolution every 18.61 years. First of all, is that "normal" years or sidereal years? Would anybody be able to give me the precise number of hours for this precession so I won't have to guess how long the days or years are that are being used? Next, does anybody know what the "unaccounted for" precession is? That is, once the amount of precession due to the motion of the Earth and sun and planets has been accounted for, how much is left? And how much is accounted for by GR?

hhEb09'1
2006-Sep-12, 05:29 AM
The precession of the moon's orbit is one revolution every 18.61 years. First of all, is that "normal" years or sidereal years? 18.61 normal years differs from 18.61 sidereal years by only a quarter of a day, which is less than .001 year. So, to the two decimal points given, 18.61 normal years equals 18.61 sidereal years.

grav
2006-Sep-12, 07:14 AM
18.61 normal years differs from 18.61 sidereal years by only a quarter of a day, which is less than .001 year. So, to the two decimal points given, 18.61 normal years equals 18.61 sidereal years.
But I thought sidereal years had one extra day, so that 18.61 years would differ by 18.61 days between sidereal and normal. :confused:

grant hutchison
2006-Sep-12, 10:27 AM
But I thought sidereal years had one extra day ... :confused:That's sidereal days: there's one more of them in a year.
A tropical year (which is what I presume you mean by "normal") is 365.2421897 days; a sidereal year is 365.25636 days. The difference is because of the precession of the Earth's axis.
The moon's node precesses in 6798 days, which comes out to 18.61 in either currency.

Grant Hutchison

grav
2006-Sep-12, 10:49 AM
That's sidereal days: there's one more of them in a year.
A tropical year (which is what I presume you mean by "normal") is 365.2421897 days; a sidereal year is 365.25636 days. The difference is because of the precession of the Earth's axis.
The moon's node precesses in 6798 days, which comes out to 18.61 in either currency.

Grant Hutchison
Okay. Great. But that is a twenty-four day as well, right? Not that a few minutes makes that much difference, I suppose. I guess the more important question is how much of the moon's precession is unaccounted for through regular Newtonian means?

antoniseb
2006-Sep-12, 12:27 PM
how much of the moon's precession is unaccounted for through regular Newtonian means?

What are you searching for? The Earth-Moon-Sun-(Jupiter) system is fairly complicated what with tides and the Moon's uneven mass distribution. It would be pretty hard to tell what is and isn't accounted for through regular Newtonian means. Are you hoping to spot some evidence of relativity or MoND?

hhEb09'1
2006-Sep-12, 01:12 PM
Are you hoping to spot some evidence of relativity or MoND?grav has his own theory (http://www.bautforum.com/showthread.php?t=45728) that he is working on

antoniseb
2006-Sep-12, 01:43 PM
grav has his own theory (http://www.bautforum.com/showthread.php?t=45728) that he is working on
Ah yes, I remember now. Well, my point is that at the scale of the limits of measurement, the Moon's motion is not a clean system for basing theories on.

grant hutchison
2006-Sep-12, 02:10 PM
Okay. Great. But that is a twenty-four day as well, right??It's a 24-hour day.

I guess the more important question is how much of the moon's precession is unaccounted for through regular Newtonian means?I've never heard that there was any concern over the moon's orbit requiring unexplained physics. It would be big news, don't you think?

Grant Hutchison

grav
2006-Sep-12, 07:20 PM
I've never heard that there was any concern over the moon's orbit requiring unexplained physics. It would be big news, don't you think?

Grant Hutchison
Wouldn't there be some part of it that is due to precession caused by some form of GR that is different from the Newtonian value? If not, then why would it only matter in some applications and not others? Would the value just be too small for the moon to be detectable? I get a value of .4461 arc-seconds per year due to the Earth's spin, but that may vary depending on several factors. Those factors would be the overall angle of the orbit of the moon to the Earth's rotation and that the mass of the moon is semi-close to that of the Earth and so the two orbit each other. There may be others, like that the moon also has an epicycle in respect to the sun and Jupiter. The different causes of the precession seem to have been worked out in detail for the first few planets. I just thought it might have been for the moon as well. The entire precession of the moon would be 360 degrees/18.61 years=19.344 degrees per year, so the value for the Earth's spin with no other factors considered yet would account for about 6.4*10-6, or 1/156000 of it.

grant hutchison
2006-Sep-12, 07:28 PM
Wouldn't there be some part of it that is due to precession caused by some form of GR that is different from the Newtonian value?Perhaps we're at cross-purposes. Your original question was
... once the amount of precession due to the motion of the Earth and sun and planets has been accounted for, how much is left? And how much is accounted for by GR?I'm assuming that GR falls into the "accounted for" category, and I'm suggesting that there is no "unaccounted for" category, or it would considered something of a major puzzle in celestial mechanics and we would have heard about it.

Grant Hutchison

grav
2006-Sep-12, 07:47 PM
Perhaps we're at cross-purposes. I'm assuming that GR falls into the "accounted for" category, and I'm suggesting that there is no "unaccounted for" category, or it would considered something of a major puzzle in celestial mechanics and we would have heard about it.

Grant Hutchison
Well, I don't want the amount that is predicted by GR, unless, of course, it matches the discrepency. I want the total amount of precession minus that much which can be accounted for by purely Newtonian means. You know, like they did with Mercury so long ago.

publius
2006-Sep-12, 10:16 PM
Grav,

There are controversial claims that one can see GR gravitomagnetic effects in the moon's motion. The lunar ranging data using the reflectors left by Apollo allow for very precise measurments of the moon's motion. I'll see if I can find it, but basically someone was claiming he had found the gravitomagnetic effect in accord with GR's predictions, thus confirming it (and ahead of GPB results -- there may be some desire to be the "first" to confirm it, so some want to find data to do it before the GPB team does :) ).

But some say the experimental errors are much larger than they think and thus the lunar data cannot confirm GR. I saw that somewhere recently and I'll see if I can find it.

The gravitomagnetic effects on an orbiting body will be complex. Let the body rotate as well as orbit and it gets even more complex. Basically, "everything will precess". :) There is a precession of the perigee of elliptical orbits, just like the other, much larger GR effect (different mechanisms mind you, but both causes the ellipse to rotate around).

If the plane of the orbit is not normal to the spin axis of the gravitomagnetic source, the orbital L vector will precess as well. The orbital plane will "wig waggle" a bit -- there's a name for this I forget. This orbital L precession would be properly called "Lense Thirring", but the precession of periapsis would be called something else.

And then finally, if the orbiting body is also spinning, it's own spin L vector will precess a bit. If the bits of matter making up the spinning planet were free to move as they would, there would be similiar precession of periapsis for all them about the center. In a semi-rigid body, this would introduce more shape distorting effects.

But since the solar system's gravitomagnetic field is so vanishingly small, these effects are so small they are almost impossible to measure. It was considered flat impossible until recently. See the paper I posted in Nduriri's now locked "Pioneer Anomaly" thread. The shape distorting part is probably well below any possible threshold of measurement.

-Richard

publius
2006-Sep-13, 02:37 AM
Grav,

http://physics.ucsd.edu/~emichels/Gravitomagnetism.ppt

That is PowerPoint slide show on B_g, which mentions the effects on the Moon's orbit.

http://arxiv.org/PS_cache/gr-qc/pdf/0507/0507041.pdf

This is a paper by Iorio. He mentions that he has problems with the accuracy of LAGOES satellite data, not the Lunar Ranging data. This was done by Nordtvedt.

ETA: Here is Nordtvedt's LLR paper: http://arxiv.org/PS_cache/gr-qc/pdf/0301/0301024.pdf

This gets pretty darn hot and heavy. He is looking at a broad class of theories of gravity, and all the contributions to acceleration of bodies from them. Look at his A-F list of terms for the general N-body problem. He considers terms arising from the variation of inertial mass and gravitational mass.

Anyway, I'd say the motion of the moon has had the heck analyzed out of it.

Now, see what GR has backing it up, and why "mainstreamers" get a little bent of shape when one someone attacks GR? :)

-Richard

grav
2006-Sep-13, 04:23 AM
Thank you very much, Publius. Those should be very informative. But I think Antisoneb is right. The motion of the moon is way too complicated. I wouldn't even be sure which part of its motion I was trying to match up. There are just way too many considerations involved. I think I'll try something a little simpler.