[LEAPSECS] Mean ... Orbits

Steve Allen sla at ucolick.org
Thu Feb 3 01:09:08 EST 2011

On Wed 2011-02-02T22:03:09 -0500, Finkleman, Dave hath writ:

> Now that I've exposed my lack of understanding, the conclusion is that

> the "mean" solar second is the outcome of a statistical estimation of

> governing state variables fitted against observations. Which "means"

> that it is really not necessarily an average over time.

Mean Solar Time is a term as much out of place as Universal Time.

It is "mean" in a sense of a meaning that started eroding fast
during the 18th century. It is mean over the period of a year,
mean because it smooths over the annual variation caused by
obliquity and eccentricity.

It is mean because until the 17th century there was no concept that
any chronometer could be more stable than the diurnal passage of the
stars overhead.

It is mean because the combination of Mean Solar Time and the
Gregorian Calendar produce the conventional phenomenon of the
Analemma, and with those a civilization cah synchronize its
agriculture with the seasons as indicated by the calendar and its
civil events such as the expiration of fire insurance policies with
the clocks that are reset to 12:00 each day when the shadows point due
north (or south). The ability of a camera controlled by clock and
calendar to produce an Analemma is a tribute to human conventions of
what "mean" means.

It is mean because even with the math of the 19th century showing how
the orbits of planets varied with time it was still possible to define
mean longitudes with simple polynomials.

It is mean because a uniformly rotating earth would have a
conventional concept of Mean Sidereal Time where the mean equinox
moves uniformly across the sky. In combination with Newcomb's.
expression for the mean longitude of the sun and an expression
for nutation Mean Sideral Time becomes Mean Solar Time, and
Mean Sidereal Time can be measured to subsecond precision by
human observers who can then set the clocks such that two
telegraphers on opposite sides of an ocean know when they need
to sit down to communicate with each other.

It is a strange trick of history that the Admiralty pulled the stunt
of ruining the definition of GMT in 1925, for that triggered the IAU
to recommend the term Universal Time just as chronometers were
becoming as stable as the earth, just a few years before it became
evident that earth rotation was not constant, and therefore Universal
Time is not time in the sense of physics.

Yet Universal Time is what divides the Universal Day, and the
international agreement says that the Universal Day is a mean solar
day, and over the long term the calendar is comprised of days, and the
passage of days has been the most obvious indicator of time to
biological systems, and to human culture.

This is not an argument about "mean" but about "meaning". It is an
argument about whether units of measure whose names are the same as
earlier concepts which are not time will be allowed to invalidate the
original concepts on which they were based.

Steve Allen <sla at ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m

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