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Subject: Re: Day's lenght
From: Zorbec Legras (legras@XXX.XXX)
Date: Tue Dec 31 2002 - 15:34:58 EST
> Pierre Boucher [mailto:pboucher@XXX.XXX] 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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