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    Re: Navigation News article on leap seconds.
    From: Frank Reed CT
    Date: 2005 Oct 10, 02:31 EDT

    Jared, you wrote:
    " But you're saying  that
    the rotary landfall of the rocks would have the same "thrust" impact on  the
    earth as the thrust they've transferred through the people launching  them? I
    would have thought there was still a net thrust "that way" as opposed  to the
    distributed energies in the landing impacts."
    It's absolutey  guaranteed that there is no net effect unless your rock
    throwers can manage to  propel their projectiles at a speed above escape velocity.
    It's a simple case of  conservation of angular momentum. Since the Chinese are
    now a rather successful  space power (next manned launch coming up next
    week), let's imagine a high tech  version of this experiment...
    Suppose I blast rocks into space on  trajectories that are westbound and more
    or less horizontal. Suppose I fire  trillions of tons or rock into space at
    22,000mph (not quite escape velocity).  They will make a large elliptical orbit
    around the Earth lasting a few days and  during those few days the Earth's
    rate of rotation will be slightly higher than  it would have been otherwise.
    Assuming we didn't circularize the orbits (they're  rocks, not rockets), the
    rocks will eventually return to the Earth where they  will burn up in the
    atmosphere. Each re-entering rock will create a little plume  of hot gas and dust high
    in the atmosphere. The net effect would be a net  increase in the speed of
    the upper level winds. This will slowly damp out by  friction with lower
    atmospheric levels and eventually they will transfer their  angular momentum to the
    Earth's solid mass. Net effect? We're right back where  we started from with
    the Earth rotating at its original  speed.
    "Even if you don't like chinamen and rocks, surely you  acknowledge that
    up the earth's rotation, or slowing it down, is  simply an application of a
    suitable amount of thrust in the proper  direction."
    So suppose we set up giant jet engines on mountain peaks  around the world.
    By transferring angular momentum to the atmosphere, we could  change the rate
    of rotation of the solid Earth. But when you turn the engines  off, the
    atmosphere will gradually settle down by the same process as above and  you're back
    where you started.
    But as long as you're picturing outlandish  thought experiments, there is
    another way to use angular momentum. Think of the  figure-skater example...
    Instead of throwing rocks, we could just move closer to  the Earth's core to make
    it spin faster. Tear down the Rockies and fill in the  Grand Canyon!
    42.0N 87.7W, or 41.4N  72.1W.

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