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You wrote:
"If you want to work it out right down to the second of arc of where you are
and second of time then you must consult refraction tables and work your dip
out carefully."

Of course, it's quite pointless to work out sunrise/set times to the nearest 
second. The refraction at the horizon is variable, as is the refraction "of" 
the horizon (variable dip). And there's simply no way to predict what that 
variability will be. The nearest minute, maybe the nearest quarter of a 
minute, is really about as good as it gets.

It's interesting, well a little interesting..., that sunrise and sunset times 
have become standardized calculations with little reference to reality for 
the majority of people. The same calculation is used for inland locations as 
for locations with a sea horizon. What does it mean when the standardized 
calculated time of sunrise is 08:30, e.g., if you're living in a mountain 
valley?

-FER


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