Dear Zefram
Please see my answer to Clive D.W. Feather at https://pairlist6.pair.net/pipermail/leapsecs/2017-January/006697.html I will not argue about what you have against my GDs. You are totally right in all you say. I can only say to you, as I said to Clive: "If you don't care about Christ, and the church, I can understand why you treat all timescales alike. But if you really care about the fundamental timescale of science and society, then I don't see how you can ignore the time of the incarnation." Thanks, Preben 2017-01-11 13:23 GMT+01:00 Zefram <zef...@fysh.org>: > Preben Norager wrote: > > Astronomy then have two different "eternal" timescales, with two > >different starting points for zero: > > Many more than two. MJD, TJD, and the Julian epoch, for example, all > have some currency in astronomy, and each have their own zero point. > ("Julian epoch" is a somewhat confusing term: it refers to a linear > count similar to JD, but scaled by a factor of 365.25 such that over the > long term it is frequency locked to the years of the Julian calendar, > and with zero point chosen such that Julian epoch roughly matches the > AD year number of the Julian calendar year. The zero is nevertheless > not identical to the start of AD 0 in either the Gregorian or Julian > calendar.) > > > The one is the proleptic gregorian > >calendar, represented by ISO 8601, with the starting year zero, > > That's not really the *start* of the Gregorian calendar. It extends > back much earlier than that, and there's nothing really special about > the year zero. In general, the zero point of counting systems such as > this is a lot less special than one might imagine. It's only rarely > the start of the system's applicability. It may be a reference point, > the time of some epochal event, but often it's not even that. > > Looking at the year numbering used by ISO 8601, this was originally > devised by Dionysius Exiguus in AD 525, and he originally used it with the > Julian calendar. It's not clear in which year he intended the epochal > event to occur: it may have been -1, 0, or 1. We're not even sure > which event he intended to define the epoch: it's either the conception > or the birth of Jesus of Nazareth. We're pretty sure that he got the > computation wrong: it's currently reckoned that Jesus was born around the > year -3 in this numbering. Dionysius actually defined AD year numbering > by reference to the Diocletian era, a year numbering in common use at > the time. He defined that AD 532 was the year following Diocletian 247. > > ISO 8601 uses neither of these as the reference point. It fully defines > the proleptic Gregorian calendar. But to specify its phase it does not > refer to any event in the vicinity of AD 0, nor to the Diocletian era, > nor to any event in the vicinity of AD 532. Rather, it says > > The Gregorian calendar has a reference point that assigns 20 > May 1875 to the calendar day that the "Convention du Metre" > was signed in Paris. > > So the reference point is actually 1875-05-20. Definitely not the start > of the calendar, nor a zero date. It's much more useful than either > of those would be: it's a reference event of which we have very strong > historical knowledge. > > > But I don't understand how > >astronomy can cope with two different starting points for zero. > > We do not find any difficulty in adding a fixed offset to handle a > difference in epochs. The mathematical expressions would be only very > slightly simpler if we had the same epoch for everything. What do you > think is the difficult part? > > In fact, astronomers and engineers have repeatedly found greater value > in making the JD-like day numbers that they deal with smaller, than > in maintaining a consistent zero epoch. MJD is defined as MJD = JD - > 2400000.5, such that until AD 2132 MJDs have only five integer digits > rather than the seven of JD. (The shift from being integral at noon to > integral at midnight reflects a shift in preferences, and also entails a > change of epoch.) TJD is defined as TJD = MJD - 40000, such that during > the era of its original use (the Apollo space programme) it only used > four integer digits. > > > The > >beginning of time must be a beginning in time, > > We're not dealing with the beginning of time, or really of anything. > We are not troubled by negative numbers. (There *is* a slight convenience > in having the numbers one actually deals with all be non-negative, > as I said before, but it is no more than convenience.) > > > I don't know if that is because > >Christmas day (December 24/25), and other important days, are not the same > >JD in the julian, and the proleptic gregorian calendar, > > No, we're not at all bothered about that. > > >zero point in time must be the same for both the daily, and the annual > >continuous timescale. > > "Must"? Why? > > >The new system of GD shall like JD count the days from noon to noon. But > >the zero day shall not be JD:0. The zero day of GD shall be the day from > >-0001-12-31T12:00 to 0000-01-01T12:00. That day is JD:17210159, so my > >reform will be the removal of 17210158 days from JD, to create GD. > > It won't catch on. You're not offering any real value here. > > If you personally want a day count with a zero in the vicinity of > AD 0, maybe you'd like Rata Die. Its zero point is 0000-12-31T00. > Unlike JD, it is conventionally used with timezone-relative calendar > days, not only the days of UT and other technical time scales. It was > rather gratuitously invented for the book "Calendrical Calculations", > and has some currency among computer programmers, but I haven't seen > it used by astronomers. (Beware of errors and confusion in the book. > It actually defines three slightly different day counts, all of which > it calls "Rata Die".) > > -zefram > _______________________________________________ > LEAPSECS mailing list > LEAPSECS@leapsecond.com > https://pairlist6.pair.net/mailman/listinfo/leapsecs >
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