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Re: day of shortest twilight....
From: Marcel Tschudin
Date: 2010 Jul 1, 00:17 +0300
From: Marcel Tschudin
Date: 2010 Jul 1, 00:17 +0300
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