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    Re: Day's lenght
    From: Zorbec Legras
    Date: 2002 Dec 31, 15:34 -0500

    > Pierre Boucher [mailto:pboucher{at}LAVOILE.COM] asked:
    > > Does anybody know of a formula that would compute for a given
    > > latitude and a given date the day's length ie. how many hours
    > > from sunrise to sunset?
    
    In my first answer to Pierre I gave a simplified formula to compute the duration of half a
    day, from geometrical rise/set to/from meridian transit. The full formula to compute the
    exact duration is:
    
    Cos asd =(sin H - (sin L  sin d)) / cosL cos d
    
    asd = semi diurnal arc (from rise to noon transit, in fact the pole angle)
    
    H = altitude of the sun (- 0�53' flag time; -6� civil twilight; etc)
    L = Latitude
    d = declination (at noon)
    
    the duration of a day is twice the asd in � ' " , or 2 asd/ 15 in h mn sec.
    
    If the chosen H is 0�,  the simplified formula is used whitch is :
    
    cos asd (- tan L  tan d)
    
    Cpt Legras.
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