[time-nuts] Re: time-nuts Digest, Vol 12, Issue 15
w3iwi at toad.net
Thu Jul 7 00:06:36 EDT 2005
It was said
(3) It is called UT1 these days.
Don't be pedantic. The point is that there are organizations which depend upon
a time coordinate system which is closely linked to astronomical time. UTx/xMST
Wrong! UTC is the stepwise (i.e. with leapseconds) approximation of
the atomic time rate to the actual rotation rate of the earth. Changes
in UTC are all integral seconds.
UT1 reflects the ACTUAL rotation of the earth with respect to an
external reference frame (i.e. as would be observed with a sundial ;-)
). It includes numerous effects of changes due to global winds, ocean
circulation, the viscous goo in the center of the earth, earthquakes,
etc. that makes the actual rotation of the earth vary from A1 (and by
implication, both GPS time and the rate of UTC) by up to parts in
For an FAQ that describes the differences between the different time
systems, I refer you to http://maia.usno.navy.mil/whatiseop.html.
The actual determination of UT1 is done at levels of 10's of usec/day
by an international network of radio telescopes using a technique
called Very Long Baseline Interferometry (VLBI, sometimes called by
the practitioners Very Low Basic Intelligence O:-) ). More details on
VLBI can be gleaned at the IVS (an acronym of acronyms, the
International VLBI Service) web site http://ivscc.gsfc.nasa.gov/.
[As an aside, for many years until I retired, I headed NASA's VLBI
program -- see more about the Goddard VLBI group here. You
might enjoy some of my tutorials on how the VLBI stations get their
UTC reference time so that their measurements of UT1-UTC can be
derived at the catch-all repository http://gpstime.com ]
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