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JohnD
2004-Nov-19, 12:00 AM
All,
Today's Apod (18th November 2004 - http://antwrp.gsfc.nasa.gov/apod/ ) features new ground based (!) pics of Uranus. Spinning with a grossly tilted axis it has an equatorial ring system, just like Saturn and Jupiter. Whether the rings form from the original condensation cloud that became the planet, or from a catastrophe to a satellite, these agree with the standard theory of planet formation. But Uranus is supposed to be 'on its side' after a collision with another planetoid that 'knocked it over'.

How did the planet carry its rings with it? If they were in existence before the collision, what mechanism twists them as well? A collison would have caused a chaotic cloud of debris - why should an equatorial ring system reform? A polar ring would be even more dramatic!

John

eburacum45
2004-Nov-19, 05:25 AM
All ring systems are equatorial;
if the planet was tilted by a collision, it occured before these particular rings formed.

Evan
2004-Nov-19, 05:38 AM
An object in a polar orbit experiences no tidal effects due to rotation of the primary vs the rotation of the orbiting object when over the poles. It experiences maximum tidal effects when acending or decending the equatorial nodes. This should have the effect of very gradually dragging objects in inclined orbits into an equatorial orbit. It should in effect precess the orbit to equatorial. Proof? How thick are Saturns rings?

George
2004-Nov-19, 03:05 PM
It seems clear the main force in control is the tidal force.

I think I understand the tidal force, but allow me to test my mental handle....

Hypothetically, if a small perfectly spherical and homegenous body orbits a large perfectly spherical and homegenous body, would there be any net tidal force altering the orbit? [Assume any rotations you like but no variation (movement) in mass due to tidal forces?]

[If the answer is yes, then I don't have a good handle on tidal forces.]

[Edit: Hmmm. I forgot about GR. Make it a Newton and a GR question as frame draging is looking more likely these days]

Evan
2004-Nov-19, 04:01 PM
See this paper. (http://www.phys.lsu.edu/mog/mog10/node9.html) He mentions precession of orbital planes.

George
2004-Nov-19, 06:42 PM
See this paper. (http://www.phys.lsu.edu/mog/mog10/node9.html) He mentions precession of orbital planes.

In addition to precession, wouldn't frame dragging increase the relative velocity of the orbiting body (assuming the main body to be rotating faster than the orbital speed)?

Also, is my Newtonian-only thinking correct on tidal interaction?

Evan
2004-Nov-19, 06:50 PM
Since we aren't positive about frame dragging we don't know.

George
2004-Nov-19, 10:08 PM
Since we aren't positive about frame dragging we don't know.

I am not very fast, so I tend to ignore GR. :)

Therfore, ignoring GR, what about the Newtonian application to the hypothetical balls? #-o

Evan
2004-Nov-19, 11:56 PM
Assume that all spinning planets have a degree of oblateness. Assume a satellite orbiting at an inclination of 45 degrees. As the satellite approaches the decending node and ascending node it will experience a gravitational couple produced by the equatorial bulge of the planet that is greater to the side of the orbit that is 45 to the equator than the side that is 135. The side that is the 45 angle has more of the bulge closer to the satellite as it approaches the node and conversely on the other side as it passes the node. As the satellite is equivalent to a mass at a point on the rim of a gyroscope as it rotates about the planet it will be precessed by this unbalanced force. This will precess the orbit toward the equatorial plane. It also precesses the nodes. The nodes precess very quickly but like a gyroscope the orbit gradually "falls over".

George
2004-Nov-20, 12:04 AM
Assume that all spinning planets have a degree of oblateness. Assume a satellite orbiting at an inclination of 45 degrees. As the satellite approaches the decending node and ascending node it will experience a gravitational couple produced by the equatorial bulge of the planet that is greater to the side of the orbit that is 45 to the equator than the side that is 135. The side that is the 45 angle has more of the bulge closer to the satellite as it approaches the node and conversely on the other side as it passes the node. As the satellite is equivalent to a mass at a point on the rim of a gyroscope as it rotates about the planet it will be precessed by this unbalanced force. This will precess the orbit toward the equatorial plane. It also precesses the nodes. The nodes precess very quickly but like a gyroscope the orbit gradually "falls over".
Therefore, can I not conclude a perfect homogenous sphere with no oblateness would not have any tidal influence?

I can envision the closer half of the orbiting moon to have greater pull than the other half, but I can not get a sense of any net force affecting the orbit. It is always the "bulges" I see as causing the orbital and rotational changes.

Evan
2004-Nov-20, 12:07 AM
Therefore, can I not conclude a perfect homogenous sphere with no oblateness would not have any tidal influence?

Not counting possible frame dragging I believe that is correct as far as precession goes. I may be wrong.

In the case of tidal bulges it does also affect the inclination of the orbit. It produces what is called a "gravitational torque" the precesses the spinning bodies when the inclination is non zero. This is the case with Earth-Moon and causes a 41000 year cycle in the inclination of the Earths tilt (less than one degree).