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    Re: shortest twilight problem...
    From: George Huxtable
    Date: 2010 Jun 29, 11:18 +0100

    I had written, about the date of shortest twilight-
    |
    | > 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?
    
    and Marcel responded-
    
    | 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 is wrong (I suggest) about his first point, but absolutely right
    about his second.
    
    Yes, the length of twilight certainly does depend on latitude, but the
    question was not about the length of twilight, but the DATE of the shortest
    twilight. And that, as I see it does not depend on latitude. Does Marcel
    agree?
    
    But I was quite wrong about dating the shortest twilight to the solstices.
    I should have written "equinoxes", as he says. Indeed, I thought
    "equinoxes", checked the twilight lengths, at equinox dates, from the
    Almanac, just to be sure, and then mistakenly wrote "solstices". Sorry
    about that. These things happen. Thanks to Marcel for bringing it to my
    notice and giving me a chance to correct my words.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    

       
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