[LEAPSECS] Civil timekeeping before 1 January 1972

Richard Clark rclark at noao.edu
Fri Mar 13 16:10:09 EDT 2015

How fare back before 1972 do you want to go?

Before leap seconds, before TT, TDT, TAI. Entangled in the roots of
ET and Delta-T...

Back in the 70s and 80s there was considerable effort at JPL to improve
the models of orbital motion of the Galilean satelltes of Jupiter. The
existing theory, dating back to ~1915, was marginal for the pointing needs
of the high resolution cameras on Voyager and especially the more
demanding needs of the planned Galileo mission.

Among the data sources for developing and testing the new orbital models
were historical observations of satellite eclipses. Useable records of
several thousand historical observations going back as far as the mid
1600's have been recovered. But making use of these posed quite a problem
in converting the reported times to a modern form. Depending on the
source, the times might be expressed in apparent time, mean time, or
sidereal time.

Nothing specific to leap seconds here, just a little perspective for
thinking about the measurement and representation of time.

And for Tom van Baak--

About a month ago you relayed a question about computing the Equation of
Time to high accuracy. I mentioned that in the context of precision
ephemerides the EoT would be obtainable but usually would not be a
quantity of primary interest. Expanding an expression for EoT to higher
accuracy generally wasn't done. (of course that's not how I said it but
it's what I wanted to get across.) There is a proliferation of small cross
terms (not so much a slowness of convergence as I stated), and also below
the ~5 second level you start needing to consider the rate of change in
eccentricity of the Earth's orbit, apsidal and nodal precession, etc.
You need to reevaluate the coefficients of the angular terms, and the
angle offsets every decade or so.

Well, one of the papers that came out of this work at JPL shows a

Lieske, Astronomy & Astrophysics Supplement Series #63 (1986) pp 143-202
"A Collection of Galilean Satellite Eclipse Observations, 1652-1983"
Sections 3 & 4 discuss reduction of observed times to 'UT'. Used EoT,
delta-T, and mentions uncertainties in delta-T due to uncertainty in Lunar

Smart, "Textbook on Spherical Astronomy" shows how the EoT expansion
is developed.

Tucson, AZ

On Thu, 12 Mar 2015, Tom Van Baak wrote:

> I do a lot of timekeeping here, old and new. What time_t looked like 
> before 1972 is not a problem. Yes, civil timekeeping (before or after 
> 1972) is an interest to me. But all the older stuff arrives in the form 
> of faded paper, or JPG photos, or TXT files. I would never think of 
> trying to encode that into some 32 or 64 bit binary format.
> /tvb
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