Hello Fer et al,
Meeus mentions (as you point out as well), that this formula (1), which
is derived from the well known local horizontal altitude (h) formula (2)
and is a special case where h=0, computes the hour angle (Ho)
corresponding to the time of rise or set of the sun (celestial object).
I now see what you meant by the length of the half day arc.
(1) cos(Ho) = - tan(lat) * tan(dec)
(2) sin(h) = sin(lat)*sin(dec) + cos(lat)*cos(dec)*cos(H)
For those interested in an accurate means of calculating sunrise,
transit and sunset I'd recommend chap. 14 of Astronomical Algorithms by
Jean Meeus.
Best,
Luke
fer j. de vries wrote:
>
> > >> ----- Original Message -----
> > > > From: Roger Bailey <[EMAIL PROTECTED]>
> > >> To: <[EMAIL PROTECTED]>; <sundial@rrz.uni-koeln.de>
> > >> Sent: Saturday, September 11, 1999 6:50 PM
> > >> Subject: Re: Formula to calculate sunrise
> > >
> > >> The easy, elegant formulae that you can use to determine sun rise and set
> > >> phenomena are:
> > >>
> > >> Sunrise time t: Cos t = Tan L x Tan D
> > >> Sunrise location: Cos Z = Sin D / Cos L
> > >> Sunrise Path: Cos psi = Sin l / Cos D
> > >
> > >
>
> To all,
>
> Some messages have been on this list about the formula to calculate
> sunrise and so on.
> The first one ( Cos t = Tan L x Tan D ) in fact calculates the length of
> half the day arc.
> But in that formula must be a negative sign, thus the formula is :
>
> Cos t = - Tan L x Tan D
>
> Best, Fer.
>
> --
> Fer J. de Vries
> [EMAIL PROTECTED]
> http://www.iaehv.nl/users/ferdv/
> lat. 51:30 N long. 5:30 E
