Thanks Eugene. I'll try to rephrase this, because it would be good to have it in the FAQ. If somebody can explain it better, or more correctly, please help me!
What's up with GMT, TAI, UTC, and UT1? Before 1972, the "international time" reference was GMT. In GMT, all days have the same number of seconds. A day starts at "midnight" and has 86400 seconds. TAI is another time measuring system, in which seconds depend on "atomic time" only, instead of the Sun-Earth position. TAI days have 86400 seconds, and it's origin is in 1958 January 1. Parallel with those, there exists UT1, which is the "astronomical time". UT1 depends only on Sun-Earth position. UT1 - TAI is some fractional seconds. In 1972 UTC was introduced, in order to approximate "international time" to "astronomical time". Now, whenever the difference between UTC and UT1 is big enough, a leap second is introduced. UTC is synchronized to TAI, which means that UTC - TAI is an integer number of seconds. UTC - UT1 is some fractional seconds. > [EMAIL PROTECTED] schreef: > > http://hpiers.obspm.fr/eop-pc/earthor/utc/leapsecond. > > html > > > > "... Since the system was introduced in 1972, " > > > > The table starts in 1972. Before that, GMT was > > in use - not UT1! > > Not true. UT1 existed at least since 1958. In that year, TAI (atomic > time) was synchronized with UT1. > > UT1 is based on the observed length of the day (corrected for some > large scale effects like the movement of the poles). As it is > observation based, it is not a good timescale to base civil time on. > UT was used for that. Periodically the correspondence of UT and UT1 was > checked, and UT was corrected by changing the length of the day, and by > adding a small time adjustment to account for the accumulated > difference. > > Civil time was based on UT: GMT was UT+0000, MET was UT+0100, EST was > UT-0500 IIRC (as long as you're not in Australia). > > So the role of UT/GMT was taken over by UTC, not by UT1. > > Eugene >