Thanks very much Steve. Great info....
On 2014-01-11 10:45 PM, Steve Allen wrote:
On Sat 2014-01-11T21:43:02 -0800, Brooks Harris hath writ:
Any help getting to the bottom of this appreciated.
It's history, and it's confused. Measurement techniques were crude
and people were not cognizant that there was more than one thing being
measured. Measurement techniques are vastly improved and some people
understand better, but even the best current knowledge cannot
unconfuse the folks in the past or be sure how to interpret their
understanding using a modern vocabulary and reference frame.
NASA technical report number 70 by Hans D. Preuss of the Department
of Geodetic Science at Ohio State University "The Determination and
Distribution of Precise Time" is relevant to read to see how badly
confused the situation was in the 1960s
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670028967_1967028967.pdf
NIST has many of the old NBS publications scanned and online at their
website, and many of the announcements of rationales and dates when
decisions were made to change the radio broadcasts are scattered among
those. Their publication with most dense collection of such facts is
NBS Monograph 140 which can be found at
http://digicoll.manoa.hawaii.edu/techreports/PDF/NBS140.pdf
But nobody is going to reset their clocks based on a new understanding
of when an epoch was nor what kinds of seconds were being counted.
Tabulating historic differences between the values of various time
scales is of little relevance to the decision before the ITU-R.
How they handle the leap second issue will assert whether humanity has
any intent of keeping the meaning of the word "day" to be based on the
rotation of the earth.
Yes. Its only relevant in substantiating the standards provance in the
interest of completeness.
The specific question I was trying to get at was about the 1958 origin
of TAI.
I had said "So that essentially establishes a proleptic TAI timescale
from 1958-01-0100:00:00 (TAI) to 1972-01-01T00:00:10 (TAI)."
And Warner said "I don't think TAI is proleptic during that time."
I had seen refernce to the fact the 1958 origin was retroactively
declared, and this might throw light on why there is a gap in the
TIA/UTC tables between 1958 and 1961. So I was hunting for the actual
statement in the standards.
And I think I've found it in the material you sent. (thanks again, I've
been hunting for that for too long.)
TIME AND FREQUENCY:
Theory and Fundamentals
ANNEX l.A
DEFINITION OF THE SECOND AND TAI
l.A.2. Recommendations of the 5th Session
of the Consultative Committee for the
Definition of the Second
RECOMMENDATION S 4 (1970)
Mise en Pratique (Putting into Practice) of
International Atomic Time
4. The origin of International Atomic Time is
defined in conformance with the recommendations
of the International Astronomical Union (13th
General Assembly, Prague, 1967) that is, this scale
was in approximate agreement with 0 hours UT2
January 1, 1958.
So this suggests that the TAI origin was indeed retroactively declared,
although it seems there was unofficial agreement about it as far back as
1961.
So I'm not sure which part of TAI might be called proleptic, or if its
useful to characterize it that way. But the history seems to explain the
gap in the TIA/UTC tables between 1958 and 1961. The 1958 origin was put
in place when there was enough information and agreement to declare it
as such. 1961 is the start of the accumulation of data.
-Brooks
--
Steve Allen <[email protected]> WGS-84 (GPS)
UCO/Lick Observatory--ISB Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
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