# Thread: Precession of nodes and orbit around GC

1. ## Precession of nodes and orbit around GC

I was thinking about the orbits of stars around the galactic center, and how I had read somewhere that they bob up and down like a horse on a carousel. At first, that description seemed strange, but it occurred to me that since the galaxy is essentially a disc, when a star is high above the plane it will tend to be gravitationally pulled back down, thus cycling above and below the galactic plane several times during its orbit.

But I never did understand the exact mechanism by which the ascending node and perigee of the Moon would precess due to solar perturbations, nor for that matter why those of LEO satellites precess due to the Earth's obliquity.

As I think about it though, it seems that if a body were bobbing up and down in its orbit, and this up & down motion were just slightly faster or slower than the orbit itself, what you would actually have is none other than an inclined orbit with a precessing or processing node.

So, my question is: is it that the pull of the Sun on the Moon's orbit "averages out" into a plane of gravitational pull, and would the formulae that calculate an LEO satellite's precessions also work for those of the Moon and of stars circling the galaxy?

2. yes.

3. There is more to this than you might have thought. Finding the data required to calculate all that goes into this solar systems orbital path of the galactic center is farther complicated by the notion that we are unable to calculate the amount of mater in this galaxy with great accuracy. Some of it is 'dark' or, just unseen by us. The Moon is easy. We know a great deal about it so it has a very predictable orbital path. The planetary orbits are almost as uncomplicated. It is do able. So your question regarding the procession through the galactic plain is a little out of our reach as yet.

4. Order of Kilopi
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Originally Posted by umop ap!sdn
So, my question is: is it that the pull of the Sun on the Moon's orbit "averages out" into a plane of gravitational pull, and would the formulae that calculate an LEO satellite's precessions also work for those of the Moon and of stars circling the galaxy?
You can think of a satellite in LEO as orbiting a central point mass surrounded by a gravitating ring, due to the Earth's equatorial bulge. The presence of that ring means that the gravitational forces acting on the satellite aren't precisely central, so its orbit shifts in the nodes and perigee. (So this doesn't arise due to the Earth's obliquity, but its oblateness; it would occur even if the Earth's had no inclination relative to its orbital plane.)

Likewise, gravitational forces in the galaxy are far from central. If a star stays in the galactic plane, it "sees" a force towards the centre of the galaxy; but if it shifts above or below the plane, it sees a very non-central force because of the extended mass of the galactic disc.

Grant Hutchison

5. Thanks astromark and grant hutchison! I had my terms mixed up, obliquity vs. oblateness.

To me it is a very tantalizing concept to someday be able to plot the orbits of the Sun and other stars, so as to accurately predict what the night sky will look like millions of years from now, and what it looked like that long ago. Didn't know there was that much uncertainty in the mass distribution of the galaxy.

I guess for now we have to settle for proper motion and radial velocity to predict star positions out to a few thousand years.

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