Last Friday, a member of the History of Astronomy Discussion Group (HASTRO-L) wrote the following concerning the possible redefinition of UTC:
"This is a curiosity. It looks as if it might mean that mean solar time is no longer tracked and made available. The resulting situation might recall, in a way, what used to happen in the nineteenth century, before time distribution became well-established. Some municipalities far from other sources would rely on friendly local observatories, some run by amateurs, to provide them with clock corrections for mean solar time. Some of the observers left traces of their disagreements over the details of how to do it, in popular astronomy periodicals of the time." Here is my answer: > It looks as if it might mean that mean solar time is no longer > tracked and made available. This is not true. UTC, used as civil time, is only an approximation to mean solar time. It runs in parallel with TAI, the atomic time, thus deviating more and more from mean solar time, unless a leap second is introduced. UTC can be used instead of mean solar time only if you need it with an accuracy not better than 0.5 sec or so. Otherwise you have to use UT1, which is independent from UTC and its definition and which is derived (and provided over the Internet) by the International Earth Rotation and Reference Systems Service (IERS) in the form of tables, see http://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html . See also http://www.iers.org/IERS/EN/Service/Glossary/glossary.html for explanations and links of some time-related terms. In case that UTC will be redefined so that it cannot be used any longer as an approximation to mean solar time, IERS intends to establish an UT1 service making UT1 more easily available over the Internet similarly to current Internet UTC services. However, an approximation of the mean solar time will then no longer be shown on our clocks and not be transmitted with radio time signals. Kind regards, Wolfgang Dick (This time officially as staff member of the IERS Central Bureau) --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
