If the earth did not rotate with respect to the stars, each day would be
a year long. The rotation of the earth adds (or subtracts, I’m too
lazy now to figure out which) lots more days. The siderial day (the time
of one rotation wrt the stars) is a few minutes different from the solar
day (the time of one rotation wrt to the sun).
The second point is that the speed of that year-long day varies over the
year. The earth’s orbit is elliptical, so its distance to the sun
varies which means its speed varies (Kepler’s law: the earth sweeps
equal areas in equal times). The variance between the earth’s actual
orbital angular velocity and its average orbital angular velocity
accounts for the variations you cite.
These are not two standards of time measurement, but rather two
different things you can measure with time. The time zones have nothing
to do with it except to determine the 3 days the sun is at the zenith at
noon. (and in fact, if you measure accurately, the probability that the
sun will ever be at the zenith exactly at noon at any given spot on
earth is zero; usually it will happen just before noon on one day and
just after noon on the next, or vice-versa).
--Barry
On 9 Dec 2018, at 19:35, Russell Standish wrote:
Being a late riser, and a darkness hater, I regard December 7 (the day
after
St. Nicholas’s Day, by the way) as the first sign of spring,
because it is the
day that the afternoons start getting longer. The shortest morning,
by the
way, appears to occur on January 7, One of 3 days in the year when
the sun is
at the Zenith at noon. In other words, noon is moving away from
sunset faster
that the setting sun is moving toward the horizon so the sun starts
arriving
later on the clock. Or something like that. The way I put it
implies two
standards of time measurement and I cannot think what the second one
is.
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