Reversal of Venus: Mystery of the Solar System

[EMAAN]

planet venus?

i read sometime ago that the venus revolves in opposite direction around the sun as compared to other planets.please tell me is it true? if yes then WHY?

 

[palvinder]

YES, venus revolves in opposite direction .

 

[EMAAN]

but why please explain? what is special in venus?

 

[Math Is Hard]

Just to clarify, let's not confuse "revolve" and "rotate" here:

If viewed from above the Sun's north pole, all of the planets are orbiting in an anticlockwise direction; but while most planets also rotate anticlockwise, Venus rotates clockwise in "retrograde" rotation. The question of how Venus came have a slow, retrograde rotation was a major puzzle for scientists when the planet's rotation period was first measured. When it formed from the solar nebula, Venus would have had a much faster, prograde rotation, but calculations show that over billions of years, tidal effects on its dense atmosphere could have slowed down its initial rotation to the value seen today.

http://en.wikipedia.org/wiki/Venus

 

[DaveC426913]

I believe the truth is even more mundane.

Relative an external frame of reference such as the stars, Venus rotates in an anticlockwise direction, just like all the other planets. It's just that its day is so long that it is actaully longer than its year, meaning that, by the time it has made one complete counterclockwise revolution around the sun, it has not quite turned on its own axis one counterclockwise rotation.

 

[SpaceTiger]

Relative an external frame of reference such as the stars, Venus rotates in an anticlockwise direction, just like all the other planets.

I don't think this is correct. We discussed Venus' retrograde rotation at great length in this thread:

https://www.physicsforums.com/showthread.php?t=119953"

 

[DaveC426913]

We discussed Venus' retrograde rotation at great length in this thread

We did, though I don't think we reached a consensus.

 

[SpaceTiger]

We did, though I don't think we reached a consensus.

Unless I'm missing something, you were the only one that held the viewpoint that Venus had prograde rotation from an external reference point, but I don't see any evidence to support that.

On the other hand, the negative sidereal period that Janus mentioned and the fact that Venus' solar day is shorter than its sidereal day both suggest that the rotation is retrograde from an external reference point.

 

[LURCH]

What they teaches in all them there text-books is that Venus' rotation is opposite that of the other planets (and slower than its orbital period). The main theory right now as to the cause is that Venus was struck by another object during the early period of system formation.

 

[tony873004]

...The main theory right now as to the cause is that Venus was struck by another object during the early period of system formation.

I would have guessed the opposite. That it demonstrates that Venus was not struck. If the small objects that coalasced into Venus had a net bias of 0 in their directions and velocities, then Venus should form with no spin. But the net bias is not 0. Objects on the inner track travel a little faster than those on the outer track which would create a slight clockwise bias, which Venus is currently observed to have.

Earth on the other hand needs a collision to explain the angular momentum.

I guess my thinking doesn't explain why 6 of the 9 planets have significant prograde rotation.

 

[Andre]

Doesn't work either; think in terms of momentum and spinning skaters pulling their arms in. If Venus was formed from dust particles in a orbit around the proto sun, and their angular momentum remained constant, the total angular momentum of the planet would be the original sum of momentums of the orbits of all particles plus the momentum in relationship to it's centre of gravity where the arm pull in trick of the ice skater would have caused the planet to spin up prograde during it's formation, just like Earth and Mars did.

Another approach, if the dust cloud that formed Venus would have happened to spin retrograde, the result would have to be that both the orbit and the spin of the planet were retrograde.

So a prograde orbit and a retrograde spin is weird. It has been suggested that tidal drag forces have slowed Venus down, you can add extreme tilt cycles due to resonance between obliquity and precession cycles (Correia and Lasker, Pegasus 2003) but their model required an initial spin of about 3 days while the empirical initial spin compared to Earth would have been around 18 hours as far as I recall. Some McDonald calculated that in 1963.

So impactors after all? But if Earth was to be hit by a enormous bolide, enough to affect it's spin at the point of impact, it would take 90 minutes before the other end of the planet "knew" about that impact. Meanwhile it would continue spinning undisturbed which would result in a complete break up forming another asteroid belt.

So not enough tidal drag and certainly no impactor. But I believe somebody had a weird crackpot idea about the solution here a long time ago.