[tz] suggestion: split "backward" into "backward" and "deprecated"

[Steve Allen]

On Mon 2020-05-04T19:58:42-0600 Paul Gilmartin via tz hath writ:
> On 2020-05-04, at 18:26:22, Andrew Gierth wrote:
> > (To the best of my knowledge there are no reference clocks available to
> > which one could synchronize in order to use a mean solar time, so your
> > argument seems to me to support this choice.)
> >
> Do you mean mean solar time at some arbitrary precise longitude
> or mean solar time at the Prime Meridian?  If the latter, is
> interpolated UT1 a close enough approximation?:
>     https://www.nist.gov/pml/time-and-frequency-division/time-services/ut1-ntp-time-dissemination

UT1 is not mean solar time, but it is the quantity which is
conventionally recognized as mean solar time.  In 1954 Sadler
recognized that by its existing convention UT (not then yet named UT1)
deviates from mean solar time.
https://www.ucolick.org/~sla/leapsecs/Sadler1954.pdf
Seago and Seidelmann explored this much more deeply in 2013
https://www.agi.com/resources/white-papers/the-mean-solar-time-origin-of-universal-time-and-u
and showed how an alternative formulation might be developed if
calendar days are to continue be determined by measuring rotation
of the earth.

--
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