[Avodah] A Question About Davening Vo'Sikin

[Rich, Joel]

  I thought this level of precision a bit strange, since the seconds are
not really a "significant figure" in calculated sunrise times.  The time
of the visible sunrise can vary by close to a minute (at the temperate
latitudes) from the calculated average, since the amount of atmospheric
refraction varies with the pressure and humidity.
This is besides the fact that many of the computer programs that
generate these charts don't take the altitude of the observer and the
altitude at the horizon into consideration.  (Some posekim don't
consider either, some consider only the former, and some consider both.)

I think that the time generated by a calculation that uses the average
atmospheric refraction is best viewed as an average, meaning that one
can best "play it safe" by finish keri'as shema a minute before that and
beginning shemoneh esrei a minute after that.
======================================
I'd guess the underlying question is whether chazal were defining
netz(etc) based on subjective definition (e.g. if reuven has better
eyesight than shimon, does netz occur earlier for him? Do local weather
conditions change the time of netz?) or is netz really defined
objectively and  we try to approximate as best we can in each generation
based on the tools and knowledge we have.
KT
Joel Rich
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