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Rotation of earth

  1. Dec 28, 2011 #1
    the conservation of angular momentum applied to earth as well right? but doesnt earth's rotation go through seasonal variations, is the angular momentum still conserved?
     
  2. jcsd
  3. Dec 28, 2011 #2
    The conservation of angular momentum applies to any system with no net torques.
     
  4. Dec 28, 2011 #3

    D H

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    Those external torques are however very small. The largest of these external torques causes the orientation of the Earth's rotation axis to precess over a 26,000 year period. This lunisolar precession changes the axis of rotation, but not the rate of rotation. Tidal friction does change the Earth's rotation rate, but this is extremely slow. Length of day was about 21.9 hours 620 million years ago. Over the course of a year, or even a century, this change in length of day is very small.

    After accounting for all known external torques, there is a very observable seasonal variation in length of day. So what explains this?

    The answer is simple: Conservation of angular momentum. The name of the conservation law is "conservation of angular momentum", not "conservation of angular velocity". If the mass moves about with the seasons (and it does), conservation of angular momentum dictates that angular velocity must change as well.

    Geophysicists who study the Earth's rotation divide the Earth as a whole into four primary components: The Earth's solid and liquid core, the mantle and crust, the oceans, and the atmosphere. Transfer of angular momentum between the core and mantle is a very slow process; it does not account for those seasonal variations in length of day. It is transfer of momentum between the mantle+crust, the oceans, and the atmosphere that accounts for these season variations.

    The largest contributor is the atmosphere. Cold air is more dense than is warm air at the same pressure as the cold air. This means there are seasonal exchanges of atmospheric mass between the equatorial regions and the polar regions. Similar effects happen within the oceans. If the northern and southern hemispheres were symmetric these seasonal variations would make for a biannual variation in length of day. The uneven distribution of land versus ocean in the northern versus southern hemispheres makes the dominant effect of these variations annual rather than biannual.

    Yet another contributor to variations in length of day is hydrological effects. There's a seasonal transfer of water between equatorial regions and northern regions in the form of snow and rain. Because the southern hemisphere is mostly ocean and because Antarctica is mostly desert, this seasonal transfer is much diminished in the south.
     
    Last edited: Dec 28, 2011
  5. Dec 28, 2011 #4
    You should look at this wiki page for seasonal variations in the solar day, which roughly is the time for the sun to be directly overhead from one day to the next. http://en.wikipedia.org/wiki/Solar_time
    The change in solar day from season to season can be explained by the orbit of the earth not being an exact circle, but rather an ellipse; and the tilt of the earth's axis of rotation.
    The variation in solar day ( in seconds ) can accumulate throughout the season to give a variation of up to 16 minutes from the mean.

    You can also check up on sidereal time which is time for an earth rotation measured relative to the stars:
    http://en.wikipedia.org/wiki/Sidereal_time

    This one has a little graph on the right showing the variation:
    http://en.wikipedia.org/wiki/Equation_of_time

    None of these effects have anything to do with change in angular momentum of the earth's rotation about its axis or revolution about the sun. As mentioned, in previous posts, that would require a torque, and /or changes to the distribution of mass of the earth.
     
  6. Dec 28, 2011 #5

    D H

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    It's best to qualify that "solar day" with the word "apparent." There's also a "mean solar day," which is nearly constant.

    But that is a good thing to notice. Since Tricks67 asked about rotation, I assumed that the question was about seasonal variations in length of day (LOD). LOD is defined as one mean solar day (UT1) less 86,400 seconds. Here are a couple of plots of length of day from the International Earth Rotation and Reference Systems Service (IERS, www.iers.org): [Broken]

    http://www.iers.org/SharedDocs/Bilder/EN/Variations__in__the__duration__of__the__day,property=default.png [Broken]


    http://www.iers.org/SharedDocs/Bilder/EN/Excess__to__86400s__of__the__duration__of__the__D,property=default.gif [Broken]
     
    Last edited by a moderator: May 5, 2017
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