Why not investigating an thought example of two relatively big bodies rotating around each other in perfectly circular orbit? Lets also assume that these bodies are rotating around their own axis with no inclination.
The retardation means that one body will feel the force of the other assuming the other body is slightly more remote and even more slightly ahead in its orbit. What would that lead to in orbit changes? The tidal force elongates the bodies to prolate spheroids. Physical properties of the bodies are assumed so that the elongation due to tidal force is pointed ahead in the direction of rotation. This shape shift from a perfect sphere makes a net extra force increase between the bodies, since the closer half of the body is attracted more than the remote part. In addition to this there is a torque on the spheroid since the closest half of the spheroid has a stronger force than the remote part. Do I need to make a picture? Is there a good program for writing curved arrows? I tried OpenOffice Draw but it lacked curved arrows (or I never found out how to do them). David David Jonsson, Sweden, phone callto:+46703000370 On Wed, Mar 3, 2010 at 4:27 PM, Stephen A. Lawrence <[email protected]> wrote: > I don't really understand how tidal and retarded effects can cancel. > > Tidal effects are only detectable on extended bodies, and the "tidal > force" generally (always?) has nonzero divergence. > > On the other hand, the retarded gravity effect typically manifests > itself as a rotation of the acceleration vector versus what you'd > calculate given the (calculated) current position of the gravitating > body. It's detectable by its action on point particles, which don't > accelerate as Newtonian theory would predict. > > But the divergence of retarded gravity is still zero where the mass > density is zero. > > So, again, I don't see how they can cancel. > > I also question this assertion: > > [DJ:] > > If either of these effects are acting on a body its orbit will become > > unstable. > > I don't understand what you mean by this. Tidal forces act on all > planets, without exception, if they're in orbit. But as long as the > planet's own gravity is larger than the tidal forces acting on it, that > doesn't result in instability in its orbit. In fact the effect of tidal > forces can be to "lock" a planet into a particular orbit and rotation > rate -- in other words, tidal forces can make the orbit *more* stable. > > > On 03/01/2010 04:13 PM, David Jonsson wrote: > > Hi > > > > My own posting today on Usenet. > > > > I would be glad if someone could help me with how to calculate this for > > some examples to see if there is any reason in it. > > > > David > > > > David Jonsson, Sweden, phone callto:+46703000370 > > > > > > ---------- Forwarded message ---------- > > From: *David Jonsson* <[email protected] > > <mailto:[email protected]>> > > Date: Mon, Mar 1, 2010 at 6:41 PM > > Subject: Can retarded gravity be counteracted by tidal acceleration? > > To: [email protected] <mailto:[email protected]> > > > > > > Could the accelerating tidal effects > > http://en.wikipedia.org/wiki/Tidal_acceleration > > be cancelled by retarded gravity effects > > http://en.wikipedia.org/wiki/Speed_of_gravity ? > > > > If either of these effects are acting on a body its orbit will become > > unstable. Tidal forces are almost always accelerating and the retarded > > gravity effect is decelerating an astronomical body. Maybe there are > > conditions when the two effects balances each other and maybe these > > conditions form the structure known as the Titius series which is > > purely empirical and not yet explained > > http://en.wikipedia.org/wiki/Titius–Bode_law > > . > > Exoplanets give new cases to test this idea. Moons, rings and pulsars > > could also be tested against this idea. > > > > David > > > >
