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That's really a nice and easy way to calculate it. Before the age of
computers people were more motivated to find such practical rules,
like e.g. the one Gauss found for the date of Easter.

This rule relates however only to twilights where the sun actually
reaches an altitude of -18 deg or lower. It ignores e.g. the yellow
data points in the graph for a location at 65 deg North.

For the North Pole I got the following results for this year:
The sun has reached there an altitude of -18 deg on 28 JAN. Assuming
for the atmosphere nautical standard conditions, the moment of sunrise
would then have been on 18 MAR. The duration of the twilight
corresponds to 48d 13.5h. What could the 26 FEB now relate to, which
results from this rule? May be the middle date? According to my
calculations this would then be 22 FEB, still not too bad for such a
basic rule.

Marcel

   

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