[LEAPSECS] alternative to smearing

Wed Jan 11 09:41:04 EST 2017

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 <zefram at 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
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