[tz] scope of tz database, scholarship

[Steve Allen]

For the sake of the scholarship I would like to see a modification of
the current text that reads
  As for leap seconds, we don't know the history
  of earth's rotation accurately enough to map SI seconds to historical
  solar time to more than about one-hour accuracy; see Stephenson FR
  (2003), Historical eclipses and Earth's rotation, A&G 44: 2.22-2.27
  <http://dx.doi.org/10.1046/j.1468-4004.2003.44222.x>.

A more complete reference to the accuracy of historical Delta T is in
Historical values of the Earth's clock error Delta T and the calculation of eclipses
Morrison, L. V. & Stephenson, F. R.
Journal for the History of Astronomy (ISSN 0021-8286), Vol. 35, Part 3, No. 120, p. 327 - 336 (2004)
as can be seen at
http://articles.adsabs.harvard.edu/full/2004JHA....35..327M
where, in particular, Table 1 gives uncertainty estimates
back nearly 3000 years.

For the oldest data there is an addendum
Addendum: Historical values of the Earth's clock error
Morrison, L. V. & Stephenson, F. R.
Journal for the History of Astronomy (ISSN 0021-8286), Vol. 36, Part 3, No. 124, p. 339 (2005)
http://adsabs.harvard.edu/full/2005JHA....36..339M

In addition to that, for all practical purposes, civil time prior to
1972 did not use SI seconds, but rather mean solar seconds, and
for dates prior to 1955 there cannot even be an argument.
Therefore in the extended range of the tz database there is an
arguably unspecified point when the kind of seconds counted changes
from mean solar (UT) to atomic (SI).

--
Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
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