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Earth's Spin

  1. Aug 1, 2004 #1
    Why does the earth spin and second why do you not see it in outerspace when astronauts look at earth, I would think they would see it spin the 30 KMs as second like it does??

    I have not idea so maybe I am wrong maybe they do see it.

    Cheers Woody
  2. jcsd
  3. Aug 1, 2004 #2


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    To a distant observer who was stationary relative to the Sun, the Earth's rotation rate would look no more or less impressive than the rotation rate of a sundial's shadow does to those of us living on the Earth.
  4. Aug 1, 2004 #3
    First of all, the Earth spins at about 460 m/s at the equator. Not 30 Km/s. Simply too slow to notice if you were looking at the Earth from the moon for example. Secondly, since the early 70's the highest astronauts have flown is under 1000Km above the Earth. (I'm pretty certain of this). Generally they stay at about 250-350Km above the Earth. At this altitude, they orbit the planet once every 90 minutes, or at about 7.5 Km/s. When watching video of the space shuttle in orbit, for instance, if you look at the Earth, you'll notice it zipping by pretty quickly.

    For your first question though, someone else could probably answer it better.

    I just realized that the 30 km/s you said refers to its orbit about the sun. In which case, no astronaut has ever left Earth orbit.
    Last edited: Aug 1, 2004
  5. Aug 1, 2004 #4
    Really I didn't know that but it makes sense, earth orbit i imagine would be quite large.
  6. Aug 1, 2004 #5
    but why spin, why do all these planets spin,

    why does our galaxies spin

    why do cluster and super cluster of galaxies spin?
  7. Aug 1, 2004 #6


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    Spin is relative to the observer. You spin, I spin, we all spin. A reference frame thing.
  8. Aug 1, 2004 #7
    I understand that, but we know they spin because of the bulges in them or at least some theories contend that.

    Humbly I ask you for a better answer. I am not saying your is wrong but help me with the spin here I am satified with frames of reference. Why do we spin. What causes it may just be the better question. k

    thanx though
  9. Aug 1, 2004 #8


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    Woody, my best guess is conservation of momentum. If you can't move forward, the only alternative is to spin. Marcus and Nereid could explain it better than I can.
    Last edited: Aug 1, 2004
  10. Aug 1, 2004 #9


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    That everything has SOME spin doesn't require much explanation. The alternative is that all the forces in the universe lined up exactly, which seems unlikely. Because if you shove something here, and something else shoves it there, and the two forces don't line up, then the object will turn. And once it is started turning it will continue doing that until some other pushes stop it. That's the physical law of consrvation of angular momentum, which all physical theories obey.

    What does require explanation is when you find a system of objects all spinning in the same direction.
  11. Aug 1, 2004 #10
    Following on from selfAdjoint's excellent way of thinking about it ... it's amost impossible to hit any kind of ball without giving it some spin.
    Last edited: Aug 1, 2004
  12. Aug 1, 2004 #11
    OK I have two cents too.

    First of all, let's talk system. if a system does not interact in any way with its environment, then we assume that certain mechanical properties of the system cannot change. This is a basic axioma. We just have to accept that or get crazy thinking about it. These properties are sometimes called "constants of the motion". and these are considered to be "conserved". The conservation laws can be considered to be the most fundamental principles of mechanics. In mechanics, examples of conserved quantities are energy, momentum, and angular momentum. Angular momentum being defined as the product of angular inertia and angular velocity. The conservation laws are exact for any isolated system.

    So conservation of angular momentum is the thing we have to accept and what must be the basis for why the Earth is spinning. Now a next bold step might be the supposition that before the big bang, there was no angular momentum and after it, there was no angular momentum as well. However during the expansion on the universe all particles started complicated dynamic motions and interacting gravitationally this would causing spinning, bodies are bumping into each otherm, causing spinning as well, but in a way that the sum of all angular momentums in the universe remained zero. However the individual galaxies, dust clouds and whatever matter could have a non zero angular momentum.

    If our solar system was formed out of a dust cloud with a very slight overall angular momentum as mentioned, then the ice skater trick does the rest to speed up a planet spinning as it forms by increasing density and decreasing the distance between the particles, hence maintaining its individual total angular momentum.


    So why is Venus not behaving like this?
  13. Aug 1, 2004 #12
    Conservation of momentum is the reason the planets are still spinning, but the current most accepted answer as to how they started spinning has to do with the initial formation of the solar system itself. Currently, the best model we have for the formation of the solar system in the "Nebular Model." In this model, the sun formed as material accreted in the center of a spinning cloud of gas and dust. The planets formed in a very similar way as material accreted in turbulent vortices in the nebula. The spin of the planets comes from the rotation of the vortices from which they formed.

    There are still some details of this that haven't been completely worked out, but this is the currently accepted explanation for the planets' spin.
  14. Aug 1, 2004 #13
    why isn't venus?
  15. Aug 1, 2004 #14
    Venus does spin, albeit retrograde. It's rate of rotation is one revolution every 243.01 days
  16. Aug 1, 2004 #15
    sorry if i am a lil stupid or behind but what is retrograde motion. does that just mean super slow.
  17. Aug 1, 2004 #16
    No problem woody - the essence of science is asking questions. Retrograde means the reverse of normal. On Venus, if you could see it, the sun would rise in the west and set in the east.
  18. Aug 1, 2004 #17

    wow thanx interesting. Is there anything that explains why Venus does that.

    thanx geometer
  19. Aug 2, 2004 #18


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    There are several models of how Venus came to have the spin it does. Andre (who posted earlier in this thread) knows a great deal about this, and can describe several of these well.
  20. Aug 3, 2004 #19

    I just happened to post about exactly the same subject here:


    That's basically it. I could elaborate some more, getting the thread on a higher level of exactly the same uncertainty :smile:

    I have my own hypothesis about how a fast prograde spinning Venus could end up in the present state. There is a thread about it, and I have to get the paper published somehow.
  21. Aug 4, 2004 #20
    Geometer, Nereid, and Andre thanx alot, I'm gonna look into this a lil more, but a take a lil more knowledge out of each of these posts thanx to you all, good luck with that paper.
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