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Hi Tim:

There are a number of methods now used to determine the rotation of the
Earth.
The IERS is the official organization that does this.
http://hpiers.obspm.fr/

In the past a Photographic Zenith Tube, a telescope using a pool of
Mercury as a reflector, looking at one star that passes directly
overhead was used.  Today, Very Long Base Interferometery, satellites
with corner reflectors, the corner reflector on the moon are used.

For a year or two in the 1960s the definition of a second was changed
each year to match the earth's rotation, but then the it was changed to
be based on Cesium, and has been Cesium based ever since, so for the
last 40 years or so the measurement methods have remained about the same.

Somewhere on the web is a plot of the deviation from 86,400 seconds of
the Earth's rotation for every day.  There's quite a bit of noise in the
plot and a few years ago when there was an El Nino there was a big
spike.  So weather can have a noticeable effect.  For the last couple of
days the Earth has been hit with a large solar storm (a huge amount of
energy has arrived at the Earth from the Sun).

Have Fun,

Brooke Clarke

--
http://www.PRC68.com
http://www.precisionclock.com
http://www.pacificsites.com/~brooke/PRC68COM.shtml



tjb3@IX.NETCOM.COM wrote:

>Hi all,
>
>I have been following this thread (as all others) with interest.
>
>One thing I have not seen mentioned so far (unless I missed it) was the 
possibility that the way in which the change in the Earth's rotation is 
measured has been modified; i.e., a "better" method of determining the rate 
of change in rotation has been implemented.  And this "better" method has 
determined that no leap second is required this year to compensate for the 
inaccuracies of the previous method(s).
>
>I do not know how the Earth's rotation is measured to begin with, with 
respect to leap second calculations.  So if my question is out of place, 
please excuse the diversion.  But unless one asks....!
>
>V/R,
>
>Tim B.
>Nav-L Wallflower
>
>

   

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