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 10e8. 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 References 1. http://maia.usno.navy.mil/whatiseop.html 2. http://ivscc.gsfc.nasa.gov/ 3. http://lupus.gsfc.nasa.gov/ 4. http://gpstime.com/ _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts