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Suppose you are the young Earth that is being formed in the early solar system. Let's say north is up, so left of you is the (soon to be?) sun, since the Earth orbits the sun counter clockwise. Stuff that is left of you is closer to the sun and is therefore going faster than you. Similarly, stuff to the right of you is going slower. It seems to me that by accreting stuff there is a torque on the Earth which acts clockwise. So I would expect that the Earth spins clockwise around its axis. However, the Earth is spinning counter clockwise on its axis.

What am I getting wrong?

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  • $\begingroup$ While there is another explanation and I'm not saying mine is the correct explanation, I would like to point out a flaw. As Earth was accreting in the early Solar system, assuming there were rock chunks to the left and right and that stuff closer to the Sun was going faster, this means that stuff on the left side approaches from behind and stuff on the right from the front. So far so obvious. Remember Earth gravitates and pulls things toward it. This means things give Earth a kick either from behind and to the right or from the front and to the left. Wouldn't that set up a CCW rotation? $\endgroup$ – Jim Apr 27 '17 at 12:54
  • $\begingroup$ The correct answer probably has to do with the formation of the Moon. The giant impact hypothesis makes irrelevant the rotation due to minor impacts during accretion. $\endgroup$ – Jim Apr 27 '17 at 12:56
  • $\begingroup$ I don't understand your first comment. Like you say, stuff on the left approached from behind, so this would give a kick from behind on the left, and stuff on the right approaches from the front, so this would give a kick in the front on the right side. This results in a Clockwise rotation. $\endgroup$ – Ward Beullens Apr 27 '17 at 13:02
  • $\begingroup$ The orbital velocities of the nearby objects will be fairly close to that of Earth (if all orbits are circular). But gravity will give these objects new components of velocity towards Earth. For stuff on the left, it will have new components of velocity towards the right and vice versa. From an Earth-fixed frame of reference, this will mean the stuff on the left will have a small CCW angular velocity as will the stuff on the right. Studies show that this wouldn't produce the rapid rotation periods we see, but it still becomes prograde $\endgroup$ – Jim Apr 27 '17 at 13:22
  • $\begingroup$ Then, you have to factor in material on elliptical orbits, which complicate things significantly $\endgroup$ – Jim Apr 27 '17 at 13:23
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What am I getting wrong?

You are getting a number of things wrong. First and foremost, that's not how terrestrial planets formed. Your model is a bit more applicable to the formation of giant planets, but even then what you wrote is incorrect. The gas and dust in the neighborhood of a forming planet are somewhat buoyed outward by pressure, making that gas and dust orbit at somewhat less than orbital velocity. A forming planet is disconnected from that outward pressure, making it orbit at more or less orbital velocity. A forming gas giant sweeps through that cloud of gas and dust.

Getting back to terrestrial planets, the dominant theory is that, with a sprinkle of magic (a number of unsolved issues remain, particularly the so-called "meter size barrier"), bits of dust collected into larger bits of dust, which eventually collected into pebble-sized objects, then bolder-sized objects, and so on. The near-end result was a few hundred objects the size of the Moon to Mars.

These protoplanets collided with one another in pairs to form even larger objects. The rate and orientation of the rotation of a post-collision object was very much dominated by the geometry of the collision rather than the individual rotations prior to the collision. That the Earth is rotating more or less (more or less meaning within 24 degrees) in line with the Earth's orbital plane is pure happenstance.

Ignoring that Venus is upside down, there are three other terrestrial planets that also happen to be oriented somewhat similarly. Mercury is tidally locked (strictly speaking, it's in a 3:2 resonance, but for a body with a large eccentricity, this resonance is more energetically stable than a true tidal lock). Whatever orientation Mercury had primordially has long since vanished due to torques from the Sun. The same applies to Venus. Venus is close enough to the Sun that it experiences significant torques. It, too, appears to be in a final state. With regard to Mars, it's rotational state is chaotic, with the orientation of the rotational axis varying by over 60 degrees. There's no telling what Mars's primordial state was.

That leaves the Earth. The dominant hypothesis regarding the formation of the Moon is that the last big thing to whack the Earth was a Mars-sized object. This collision happened to leave the Earth with an orientation only 24 degrees off from its orbital angular momentum axis. Extrapolating meaning from a significant deviation from zero is always dangerous. Extrapolating meaning from a sample size of one is even more dangerous.

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  • $\begingroup$ First of all, thanks for your informative answer. Okay, the fact that the rotation of the earth is more or less in the direction that the earth orbits the sun is happenstance, but what about the gas giants then? I understand that they are sweeping through the surrounding stuff at a larger speed.(Thanks for the nice explanation btw) But this does not really change anything, does it? The stuff on the inside would be slower than the gas giant, but still faster than the stuff on the outside, resulting in a net clockwise torque on the planet, right? So why do most gas giants rotate CCW? $\endgroup$ – Ward Beullens Apr 27 '17 at 18:31

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