[LEAPSECS] alternative to smearing

[Preben Nørager]

On Mon Jan 9 17:38:46 EST 2017 Zefram wrote:


"[Preben Nørager: So in a very long time-span the number of julian days can
no longer be used without cultural bias.] What do you mean by "cultural
bias" here?  Are you just suggesting that TAI, by its artificiality, is an
example of cultural bias, whereas UT is natural and independent of culture?"


No, I mean there is no timescale without cultural bias. There was a time
when science and astronomy used julian days because it was thought to be
without the cultural bias of for instance the gregorian calendar. With any
given time expressed as a fraction of a julian day the same time could be
expressed in any given calendar, and that way the julian period was thought
to be without cultural bias. In his Compendium (1906) Newcomb writes:


"The fundamental unit for measuring long intervals of time, when the
greatest precision is required, is the mean solar day, as already defined.
Taking any fixed date as a fundamental epoch, we may express any moment in
history by the number of days and the fraction of a day before or after
this epoch. One system of doing this, which has the advantage of being
continuous through all history, is that of using days of the Julian
period." (page 124)


We now know that the julian period is not all of history, and we also know
that the mean solar day is not an expression of the greatest precision. So
I don't understand why science and astronomy still uses JD and MJD. If JD
is only an expression of a calendar day, I don't understand why science and
astronomy still uses julian calendar days, and not days of the proleptic
gregorian calendar. With a simple computer program it is easy to compute
the exact fraction of days between two times in the proleptic gregorian
calendar, as a linear count of days, and for the outreach to the general
public, I think it would be good for science and astronomy, to use the
gregorian calendar which the general public understands.


I think my argument here for the civil and scientific use of the Gregorian
calendar does have something to do with the choice of underlying time
scale. The general public will never understand leap seconds, and I think
the general public shall have a time and a calendar they understand.



2017-01-09 23:38 GMT+01:00 Zefram <zefram at fysh.org>:

> Preben Norager wrote:
> >Julian days count solar days.
>
> They count whatever days they are applied to.  Originally that was solar
> days, of course.
>
> >julian days somehow count from solar noon, and that is why I wrote the
> >julian period count apparent solar days.
>
> That's a misunderstanding.  The Julian Date counts from noon of, again,
> whatever days the system is applied to.  Just as the Modified Julian
> Date counts from midnight.  The noon and midnight are not necessarily,
> or even usually, the noon and midnight of apparent solar time.  They can
> equally well be the noon and midnight of mean solar time, and indeed this
> usage with various flavours of UT is probably the most common application
> of JD and MJD.  When these day count systems are applied to non-solar
> time scales, the noon and midnight are then nominal constructs that have
> nothing to do with the sun.  It works just as well in all cases.
>
> >                                         Without leap seconds in the civil
> >time, then in a very long time-span the number of calendar days not
> >nessesarily match the number of solar days.
>
> This is true, and it is reasonable to argue as a result that when using
> a non-solar time scale we should not use day labels that were originally
> meant for solar days.  In principle, to apply JD or the Gregorian calendar
> to TAI is as nonsensical as it would be to apply them to sidereal time,
> or to Airy Mean Time (mean solar time on the prime meridian of Mars).
> But in practice that decision was made around 1950 with the development
> of Ephemeris Time, and baked into horological culture by the subsequent
> decades of use.
>
> The hypothetical TI that would succeed UTC as the basis of civil time
> occupies a strange status here, which would look a great deal stranger if
> we had not had this historical accident with ET.  TI would be essentially
> an atomic time scale, with the same nature as TAI, and so by nature one
> would think it nonsensical to use solar day labelling with TI.  But for
> it to be used as the basis of civil time, from which local timezones are
> derived which have a practical need to track the sun, it must necessarily
> have a defined correspondence to solar day labelling.  The labelling
> would be justified not by the time scale actually tracking solar time,
> but by the intent that it be used to derive local time scales tracking
> solar time.  This is a relatively unexplored view on one of the familiar
> arguments against making such a change to civil time.
>
> >                                            So in a very long time-span
> the
> >number of julian days can no longer be used without cultural bias.
>
> What do you mean by "cultural bias" here?  Are you just suggesting that
> TAI, by its artificiality, is an example of cultural bias, whereas UT
> is natural and independent of culture?
>
> >                                                                   And
> when
> >julian days can no longer be used without cultural bias the scientific
> >community will have to choose between julian days and the gregorian
> >calendar.
>
> This doesn't follow at all.  The problems, such as they are, in applying
> solar day labels to TAI's nominal days happen equally for all types
> of solar day label.  Julian Dates and the Gregorian calendar were both
> originally used for solar days, and are now both used for non-solar time
> scales as well.  They are pretty interchangeable for these purposes.
> What relevant distinction do you see between them?
>
> >                                                       I think the only
> >choice will be the gregorian calendar, because that calendar is the only
> >one in universal use.
>
> It certainly has a dominant position, used by most cultures worldwide.
> The network effect argues strongly in favour of it being used for civil
> purposes everywhere.  But that doesn't extinguish choice.
>
> Your argument here for the civil and scientific use of the Gregorian
> calendar doesn't seem to have anything to do with the choice of underlying
> time scale.  The Gregorian calendar is dominant *now* in civil use.
> It tends to be used for scientific purposes too, except in situations
> such as those seen in astronomy where a linear count of days (such as JD)
> is more convenient for computation.  How would a transition from UTC to
> TI make any difference to these pressures?  How would it even require
> a reconsideration of day labelling choices already made?
>
> -zefram
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