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George, you wrote:

> At first sight, the question appears to be an entirely trivial one. It
> seems to me (without thinking about it too hard) that there are two days in
> the year when twilight is shortest, which are the days of the solstices,
> and those dates are unrelated to the observer's latitude. Is that too
> superficial a view? Are there fine-points to the question that have quite
> escaped me?

I don't quite agree with you, George. Without going in to details, the
length of twilight is shortest near the equator and gets longer when
moving to the poles. It therefore does depend on latitude. Or, did I
misunderstand your reflections?

A further contribution comes from the sun's change in declination.
Isn't this around the equinoxes? I would therefore expect the longest
twilight for a given latitude (below latitudes where the sun in summer
may not reach 18 deg below the horizon) to be around the spring
equinox.

Marcel

   

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