[time-nuts] Re: time-nuts Digest, Vol 12, Issue 15

Tom Clark 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
, whatever.

   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 [1]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  [2]http://ivscc.gsfc.nasa.gov/.

     [As an aside, for many years until I retired, I headed NASA's VLBI
     program --  [3]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  [4]http://gpstime.com ]

   Tom Clark


   1. http://maia.usno.navy.mil/whatiseop.html
   2. http://ivscc.gsfc.nasa.gov/
   3. http://lupus.gsfc.nasa.gov/
   4. http://gpstime.com/

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