[LEAPSECS] The Debate over UTC and Leap Seconds

[Steve Allen]

On Mon 2010-08-09T17:32:47 -0600, M. Warner Losh hath writ:

> Wouldn't that also make UTC also an approximation of the mean diurnal

> motion of the sun which isn't the best indicator of astronomical time

> of day, but still a very good approximation?


Yes, to within a second.


> When the law says Mean Solar Time, and there's a number of different

> ways to compute a mean solar time, which mean solar time is the law of

> the land?  UT1?  The noisier UT2?  UTC?  They are all approximations

> of mean solar time with differing degrees of error...  Both UT1 and

> UT2 have changed how they are computed over the years.  Are the laws

> specific as to how the mean solar time is computed?


The 19th century fire insurance policies expired at noon with the
expectation that there would be ample witnesses who could testify
whether the sun was shining on the east or west side of the buildings
at the time of the fire.  That was practical then.

Laws are rarely so specific, they dare not be lest the legislators be
continually revising them as a result of changes in commonly accepted
practice.  Laws usually get no more specific than to refer to a
particular code book from some trade organization.  The details are
left to the practitioners, and disputes to judge and jury.

As 1970 approached the LORAN chains had just been converted to cesium
chronometers and the hassle of simultaneously changing frequency and
phase of the whole chain within the specs of navigation was large.
LORAN and such needed a new way.

Other navigators still needed better than one second for their manual
methods, and the difference to 0.1 s was easily encoded, so that was
the compromise.

The details of the discussions are in CCIR subcommittee documents
which are so obscure that they may only exist in Geneva.  It would
be interesting to know what other technical issues arose.  I suppose
that microwave relays/TV signals and other telecoms were mentioned.

Outside of applications like that I find it hard to believe that
there were any needs for sub-second legal timing in 1970.

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