Doug Haluza wrote:
A minor clarification:

wrote:
I am both of those and I assure you that the time of sunset is well-
defined and readily predicted for any location to a tiny fraction of
a second.

Well, not exactly to the second. Even the USNO only reports sunset time
to the nearest minute (see the "Accuracy of rise/set computations"
section in the link below). And their calculator only accepts Lat/Lon
to the nearest 1/10th of a dagree. So the error in the calculations are
+/- a minute or more. This is a technical point, but it is important to
remember that all measurements have some error.

True it is not possible to predict when the sun would be
observed tangent below the horizon, with an accuracy of
better than a minute or so. But that's a silly way to define
sunset in the first place. If the atmospheric conditions
(e.g. clouds) made make the sun unobservable, you wouldn't
say there was no sunset that day. (Though the sun would
still be observable outside of the visible spectrum)

If the "time of sunset" is _defined_ based on nominal atmospheric
conditions that moots the issue, just like defining the horizon
to be 90.8333 degrees from zenith moots the issue of the local
topography.

Those conditions can be defined as accurately as one wants.

Similarly, uncertainty in your lattitute, longitude and elevation
may also be mooted. You may be uncertain as to where
YOU are, but the estimation of the time of sunset for an arbitrary
location (which therefor you can define with arbitrary accuracy)
is uncertain only due to the variablity in the motions of the earth,
uncertainty and variablility in the orbital parameters of the Earth,
and uncertainty and variability in the apparent size of the sun.

Actually, since Universal Time is _defined_ by the orientation
of the Earth and not by atomic time, the time of sunset is
only affected by variabilty in the motions of the Earth because
they affect the place on the horizon where the sun is tangent.
That is why we have leap seconds from time to time, to
keep international atomic time and universal time in agreement
(coordinated) to within one second, though there has been
debate about discontinuing that practice.

Those leap-seconds are pretty important in orbit determination,
which gets us back to something of potential interest to glider
pilots. A GPS satellite moves moves more than a mile in one
second.

--

FF