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Planetary speeds

  1. Feb 21, 2013 #1

    I read up on astronomy for fun and I find the cosmos very fascinating

    my question is what determines the speed of a planet? whys does earth spin at its current speed where's Venus spins almost 224 earth days.

    I believe I read that the reason why the earth spins in the first place is inertia; our planet was made up of cosmic dust spinning this way and once the earth was formed it kept on spinning.

    thank you
  2. jcsd
  3. Feb 21, 2013 #2


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    Yes, that's correct. Why would you think that Venus formed in some different way?
  4. Feb 21, 2013 #3
    so what you are saying is that the cosmic dust was rotating at some speed as it was forming our planet, and the earth kept that speed ( - a few m/s) and that's why planets rotate at there current speed today ?
  5. Feb 21, 2013 #4


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    You asked why venus was different than earth. I was suggesting that you think about the possiblity that the rotating dust that formed Venus came together at a differnt rate.

    There is another factor in Earth's rotation speed. What is a big differnce between the Earth and Venus?
  6. Feb 21, 2013 #5
    I wasn't particularly asking about Venus, I was just curious of where the planets get there rotational current speeds from

    what answer are you looking for between the Earth and Venus, there are significant differences between the two sister planets;

    1) the existence of the Earth magnetic field

    2) the CO2 atmosphere Venus has

    3) the existence of active tectonic plates on the Earth

    4) LIFE

    the list goes on and on
  7. Feb 21, 2013 #6


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    Yes, but you left out the one that is important regarding the rotation of the Earth.
  8. Feb 21, 2013 #7


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    The histories of all the planets are complicated. For example, the moon is believed to have been the result of a collision between earth and another body about the size of mars. Neptune is thought to have been knocked sideways by a similar collision. Generally speaking, the rotation period of planets in the solar system appears to be related to their size - big planets spin fast, little planets spin slower. Ostensibly this is because big planets accreted more material hence acquired more angular momentum during their formation. Without knowing all the historical details, understanding the current rotational speeds of the planets is very difficult.
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