Re: slowing down the earth's spin

[Brian Whatcott <betwys@DIRECTVINTERNET.COM>]

At 08:02 PM 11/18/02, you wrote:
> Barger and Olsson page 295 claim that the earth's rotation is slowing
> down at 44 ns per day due to tidal friction. I have checked the math
> and come up with a similar answer to within a factor of 2. (Most of
> the discrepancy is probably due to what value to use for the moment
> of inertia I of the earth. I used the naive value of 0.4MR^2,
> undoubtedly an overestimate because of earth's dense core. BTW, is
> measurement of the rate at which the earth-moon distance is
> increasing, namely 0.5 cm/month, an accurate way to determine I?)
>
> Anyhow, they then say this amounts to 28 s/century. Unless I'm being
> really dumb (always possible, if not probable), this is about 4
> orders of magnitude too large.
>
> But, they claim that this is confirmed by the fact that astronomical
> events such as eclipses have been found to run systematically ahead
> of calculations based on observations over preceding centuries. Is
> this claim correct? If so, what is the correct explanation of it,
> since the 44 ns/day is not enough?
> --
> Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
>        U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
> mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/

Here's what one NASA source has to offer:

http://bowie.gsfc.nasa.gov/ggfc/tides/intro.html

Secular Tidal Braking of Earth Rotation
The secular change in the planet's rotation is a classical topic in
geophysics. It goes back some 300 years to when Sir Edmond Halley first
hypothesized that the moon was accelerating in its orbit. Most of Halley's
lunar acceleration was only apparent. It was actually the earth's rotation
slowing down, making the moon appear to accelerate. The moon does
accelerate (strictly, it decelerates), but the larger effect is the earth's
rotational braking. This braking is caused by tidal friction. Throughout
the earth's history tidal braking has played, and it will continue to play,
a dominant role in the rotation. Currently the secular change in the
rotation rate increases the length of day by some 2.3 milliseconds per
century.
To see what that means, consider this example: suppose the rotating earth
is our clock and it's been 100 years since that clock's "standard second"
was set to correspond to an atomic clock's second (which is actually almost
the case, notwithstanding that atomic clocks weren't around until 1955).
Then after 1000 days our earth clock loses about 2.3 seconds, falling
further behind the atomic clock. This long-term slowing of the rotation is
a primary reason for periodically inserting leap seconds into our
timekeeping. Of course, there are other contributors to the changing
rotation rate such as the changing atmosphere and the motions of the fluid
core; one can't blame just tides for our timekeeping difficulties.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This seems to support your position. Could the 28 second drift refer to a
rate of
28 sec/MYr ?




Brian Whatcott
  Altus OK                      Eureka!

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.