Revolution without rotation

  1. can earth revolve around the sun without rotating itself? why some planets revolve clockwise and some other anticlockwise? what decides this direction?
  2. jcsd
  3. Inertia decides the direction. Rather, the inertia of infalling chunks of matter when a planet forms decide the direction, and the inertia of the resulting spinning ball of matter (its angular momentum) keeps the direction.

    Unless, that is, some other force or drag (like tides) slowly robs it of its rotation, which is what happened to the moon. It's revolution and rotation are matched, so we only ever see on side of it--this is called "tidal locking". (The earth's slowing down, too, actually! All that hydroelectric energy we harvest from the tides? Coming out of our spin. But it's a vanishly small difference)
  4. Ok.But in the question i mean the revolution of planets around the sun, not the rotation of planets
  5. Same answer in both cases, although I think all planets orbit the sun in the same direction...
  6. Drakkith

    Staff: Mentor

    All major planets orbit the Sun in the same direction, counterclockwise as seen from above the plane of the solar system. This is due to the conditions of the gas cloud that collapsed to form the solar system. During collapse the cloud aquired net angular momentum (aka it started to spin slowly) in a particular direction and kept this throughout the collapse. The orbits of the planets are merely the final result of this effect.
  7. D H

    Staff: Mentor

    It's the other way around. The cloud collapsed to a disc because it had already acquired a net angular momentum. Look at the time spans involved. The cloud was a cloud for a long time before the protosun started to form. There was plenty of time for some passing star to spin up the cloud. The collapse was very fast compared to this long span, fast enough that angular momentum was more or less conserved during that short span.
  8. Drakkith

    Staff: Mentor

    Hmm. It had been explained to me that the random motions of the gas just happened to add up to net momentum during the collapse, however what you say does make sense. Would a static cloud aquire angular momentum from the random motions of its particles during a collapse?
  9. D H

    Staff: Mentor

    How could it? Angular momentum is a conserved quantity. There is no change in angular momentum without an external torque.
  10. Drakkith

    Staff: Mentor

    Ah ok. I think I see now. I was thinking there was already some angular momentum in the particles, but if its purely a static cloud then everything equals out for no net angular momentum. Does that sound right?
  11. DaveC426913

    DaveC426913 16,081
    Gold Member

    A large volume of gas and dust will not have a net zero angular momentum. It would be fabulously unlikely that the momentum of each of uncountably large number of molecules would add up to zero.

    So any cloud that is condensing will have some. By the time it's condensed a hundredfold, and the angular momentum is preserved, the proto-system will certainly have a net rotation, no external torque required.
  12. Drakkith

    Staff: Mentor

    Now that sounds like what I had read before about it!
  13. D H

    Staff: Mentor

    Per the dominant hypothesis (the Solar Nebular Disk Model), it would be unlikely for the cloud to have a sizable angular momentum without some external torque. There's the angular momentum problem to consider. Our sun comprises over 99.8% of the total mass but less than 4% of the total angular momentum of the solar system. The nebular model solves this problem: Stars selectively attract those particles in the cloud that have very little angular momentum. In solving this problem, it introduces a new problem. Where does the angular momentum in the nebular cloud come from?
  14. Angular momentum is imparted by the spinning star that went supernova and created the nebula. As the nebula contracts it spins faster just as a skater spins faster when he folds his arms.

  15. D H

    Staff: Mentor

    There's a problem with this explanation: It doesn't work. It is quite likely that such a nebula would be created without any significant angular momentum. Protostars don't have much angular momentum when they first form and they shed angular momentum as they form. Most of the angular momentum is left behind in or transferred to the protoplanetary disc. Most of the protoplanetary disc is blasted away when the star ignites. The star continues to shed angular momentum as it ages. By the time the star dies there isn't much angular momentum left.

    The way around this part of the angular momentum problem is that it is highly unlikely that such a nebula doesn't gain angular momentum between the time the nebular forms and the time star formation starts. Nearby stars, collisions between nebulae, and other mechanisms can add angular momentum to the nebula.
  16. I agree that a nebula can gain initial or additional angular momentum by shock waves from a nearby supernova, or the gravitational effects of a passing nearby star and the other factors you enumerate.

    There is also the hypothesis of the transference of solar angular momentum to the planets via solar magnetic field. This would supposedly have a braking effect on the sun's rotation. Do you accept that as a cause for the discrepancy between planetary angular moment and solar angular momentum?

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