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B Could we find enough mass to slow down the Earth's rotation?

  1. Nov 23, 2017 #1
    Hi, me and a friend were discussing calendars and how they go wrong. Apparently one orbit around the sun happens during, on average, 365.242189 rotations around earths axis. The persian calendar almost nails it, with a 1 sec per year error, because it is based on star observations rather than trying to fit rotations to orbits. But we started thinking could we slow down the earth rotation by adding more mass to the moon? Would it be theoretically possible to collect asteroids to the moon to the extent that earth would slow down to 365 days per year. Well that was before I learned that the earth rotation is already slowing down and will eventually be much slower. So instead.

    Is it theoretically possible to engineer earths rotation to keep a perfect 365 day per year, for our convenience? Or, maybe some other frequency like 100 days per year, as I understand, weather would get nicer, i.e. less wind, which could off course be detrimental to earth cycles in the biosphere though?
     
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  3. Nov 23, 2017 #2

    Bystander

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  4. Nov 23, 2017 #3
    Thank you, that also led me to: https://en.wikipedia.org/wiki/Tidal_acceleration that explains how the two large masses are affecting each other. However, I believe that the leap second is correcting a different problem, namely that earth´s rotational speed is constantly changing. Instead the leap year is introduced to compensate for the discrepancies between orbit and rotation that I am interested in "fixing".

    So the question still stands, is there a theoretical way of adjusting earths rotational speed?

    One explanation for the constant change in rotational speed is movements in earths crust, that gave me the idea of bulding a series of large eiffel towers with very massive donut shapes along the equator. The donut shapes could then be hoisted to slow down earth rotation and lowered to increase earth rotation. I know it is bat .... crazy.
     
    Last edited: Nov 23, 2017
  5. Nov 23, 2017 #4

    Bystander

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    Yes. Practical? Not even close.
     
  6. Nov 23, 2017 #5

    Ibix

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    In principle, a horizontally mounted rocket will do the job. Or building a flywheel as a belt around the equator. Carefully aimed asteroid strikes would also work. Carefully aimed asteroid near-misses could work by the slingshot effect (Edit - no, that would spiral us into the Sun).

    Not practical, as Bystander says.
     
    Last edited: Nov 23, 2017
  7. Nov 23, 2017 #6

    Janus

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    By "fixing" one problem, you'd be creating another. If you slowed the Earth's rotation so that 365 rotations fit exactly into one tropical year*, then the solar day would no longer be close to 86400 sec long but ~57 sec longer. Our clocks and the Earth's rotation would drift out of sync by almost a minute a day.


    * The number of days you gave was for the Tropical year, which isn't quite the same as the time it takes for the Sun to make one full trip around the Sun (365.25636 days). The Tropical year is measured from equinox to equinox, and determines the seasons, on which we base the calendar. It is different from the Sidereal year (the time it takes the Earth to orbit the sun relative to the stars), because of the precession of the Earth's axis.
    This adds an additional complication to fitting a even number of solar days to the Tropical year. The precession rate depends in part on the Earth's rotation, so if you slow the rotation, you change the precession, which changes the length of the Tropical year... It's not like you couldn't find that fit, it just makes it a bit more complex to find the right solution.
    In addition, the minimum energy needed (assuming 100% efficiency) is almost 5 times the total energy consumption of the world for 2016. It hardly seems worth the trouble. And since the Earth's orbit isn't fixed either, but changes slowly over time, you would, from time to time, have to adjust the Earth's rotation to keep it in sync with the year. (It wouldn't be a one-time fix.)
     
  8. Nov 26, 2017 #7

    CWatters

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    If you simply walk east you will slow the planet down a bit.
     
  9. Nov 26, 2017 #8

    mfb

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    We do influence the length of a day in a measurable way - with dams. They change the water distribution. If you build them close to the equator (far away from the axis of rotation) they slow it, if you build them close to the poles they speed it up. As far as I know the Three Gorges Dam has the largest effect, it makes a day longer by 60 nanoseconds or about 1 part in a trillion. Source
    As the length of a year is unchanged it means the year gets shorter when measured in days - by 1 part in a trillion as well. That is far away from changing it to 365 or even 365.25 days. And building more dams won't help long: The added mass will push the ground down, reducing the impact of more dams. In the long run the shape of Earth will always be close to hydrostatic equilibrium, which means it will have approximately the same shape no matter what we do on its surface.

    Asteroid impacts can change the angular momentum of Earth. They can hit Earth with something like 30 km/s at 1 Earth radius, about 20 times the speed of the rotation there. As rough estimate, we need x/20 times the mass of Earth to change the rotation rate by a fraction of x. To get to 365, we need a change of ~1/2000, or about 1/40,000 the mass of Earth, roughly 1/6 the mass of Ceres, or a few million times the mass of the "dinosaur killer" Chicxulub impactor.
     
  10. Nov 26, 2017 #9

    Ibix

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    I hope the asteroid is made of glue, 'cos we're gonna need it to stick the Earth back together after that...
     
  11. Nov 28, 2017 #10

    Yes, thanks for doing the math. I guess that´s what I was looking for, how far fetched my ideas were. How much mass would we need to hoist 100 meters out from the equator to slow it sufficiently? At some point, in a really distant future we might want to speed it up.

    Another question has appeared to me. What is the hen and what is the egg? These researchers attribute the synchronicity between, periods of many strong earthquakes and earths rotational speed peaks, to the fact that earth expands at high speeds and then five years after the peak when it slows down the crust contracts and causes more earthquakes. Yet, in the wiki page on "leap second", vide supra, the main explanation, for earth´s constant variation in rotational speed, is redistribution of mass in the earth´s crust. They quote 2004 indian ocean earthquake, measured to have slowed down earth rotation by 2,68 milliseconds. Surely there are other factors affecting the speed of rotation. Like El Nino (see NASA link). Could solar winds affect earth rotation? Earth is really big, so maybe it is not so much the movement of mass in the crust but rather in the mantle that is the major contributor? That could support their model.

    http://onlinelibrary.wiley.com/doi/10.1002/2017GL074934/abstract

    https://www.nasa.gov/centers/goddard/news/topstory/2003/0210rotation.html
     
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