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Re: shortest twilight problem...
From: John Huth
Date: 2010 Jun 29, 14:58 -0400
From: John Huth
Date: 2010 Jun 29, 14:58 -0400
Marcel -
Yes, I agree. I tend to first think about the mid-latitudes since that's where I live. Obviously there's no twilight in the polar regions for many months, and then when it does hit, it's very long - in fact dominated by the declination effect at the poles themselves.
The inclination angle can also be quite something - when I visited Fairbanks, the night was just twilight.
Best,
John H.
On Tue, Jun 29, 2010 at 1:55 PM, Marcel Tschudin <marcel.e.tschudin@gmail.com> wrote:
John,
Here your focus must have been on mid-latitudes; as mine when I concluded:
Earlier on you wrote:
"I think the change in declination around the equinoxes is a much
smaller effect. That's a few minutes per day, so the fractional change
in declination during the period of twilight will be a few seconds at
most."
"... it's thus a pure geometrical feature."
You indicated then that there are some interesting limiting cases and
mentioned the Poles. Indeed. At the Poles the length of the day is
governed by the change in declination and therefore also the length of
twilight. The length of the twilight is thus not only related to the
latitude and the day but also to the declination. The latter becomes
more and more important as one approaches the Poles. It's therefore
not a "pure" geometrical feature, but may be rather a "dominantly"
geometrical feature.
Marcel
