William Thompson scripsit: > Any application which seeks to calculate the difference in time between > two events recorded in UTC time needs to know if there are any leap > seconds between the start and stop time. For example, suppose you > were studying solar flares, and analyzing some data taken in 1998, > and you saw a burst of hard X-rays at 23:59:53 UT on Dec 31, followed > by a rise in EUV emission at 00:00:10 UT the next day. You'd think > that the delay time between the two would be 17 seconds, but it's > really 18 seconds because of the leap second introduced that day.
Thanks for the example. Of course it is not astronomy-specific: the same thing applies if you are calculating how long somebody spoke for in field linguistics, or the amount of time it takes a moving part to stop moving in engineering. What we are dealing with here is time-zone independent civil time. > That's a vital difference for the scientific analysis of the data. Indeed. > And yes, part of that software package includes a list of all > leapseconds added since 1 Jan 1972. Currently, my software doesn't > handle TAI/UTC conversions between 1958 and 1972, when UTC seconds > had varying lengths. Modern Unix time packages (both GNU and ADO) assume that TAI-UTC was 10 from the epoch until 1972-06-30T23:59:60 UTC. Or to put it another way, the epoch was at 1970-01-01T00:00:10 TAI. When did the TAI timescale first come into existence? One answer seems to be that TAI was born on 1958-01-01T00:00:00 UT2, which was also 1958-01-01T00:00:00 TAI. But OTOH the definition of the SI second changed in 1967 and again in 1997. What did these changes do to the uniformity of TAI? I found the following interesting statement at http://www.maa.mhn.de/Scholar/times.html : # The need for leap seconds is not caused by the secular slowdown # of Earth's rotation (which is less than 2 milliseconds per century) # but by irregular variations in this rotation and by the fact that the # definition of the SI-second is fixed on the duration of the year 1900 # which was shorter than average. -- Not to perambulate || John Cowan <[EMAIL PROTECTED]> the corridors || http://www.reutershealth.com during the hours of repose || http://www.ccil.org/~cowan in the boots of ascension. \\ Sign in Austrian ski-resort hotel
