Fred:

You should really check your facts before posting twaddle!

Clearly I should have said to ask any physicist or astronomer who is
conversant with atmospheric refraction, which you clearly are not.
Because of variations in refraction, sunset (the time when the limb of
the sun becomes invisible to an observer) may vary by several minutes
from that calculated by the accepted formula. This formula assumes a
constant refraction that is unlikely to be exact for any specific place
or time.


Mike

(also a physicist and astronomer - and who knows a sunset when I see
one!)



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.

You may have difficulty observing the position of the sun. Like,
it might be cloudy, there might be a mountain in the way, or
you might be indoors. But that is not relevent.

Any regulatory requirement should (and I daresay will) be based
on that well-defined time, or the equally well-defined times of civil,
nautical, or astronomical twilight.

http://en.wikipedia.org/wiki/Astronomical_twilight
http://aa.usno.navy.mil/faq/docs/RST_defs.html#top

Obviously you may be safer flying a few minutes after sunset in
a clear sky than a few minutes before sunset in a cloudy sky.
But that is a practical consideration.

You are correct that these definitions are based on certain
assumptions, but the point is that a rule that says you are
supposed to be on the ground by sunset, the end of civil
twilight, or whatever, should be a rule that uses the defined
time, not some local observation. That gives you a clear,
unambiguous requirement.

--

FF