I sent Steve some code for this problem, but I thought the group might enjoy
some details.
Calculating the hour angle from the azimuth angle given declination and
latitude is not as straightforward as it seems at first. The relevant equation
is:
sin h = sin lat sin decl + cos lat cos decl cos H
where h = altitude, H = hour angle from south, lat = latitude and decl =
declination.
The equation above can be solved for H, but you have to know the altitude.
Therefore, the problem becomes finding the altitude given lat, decl and azimuth.
Edmond Gunther (1581-1626), who invented a quadrant known as "Gunter's
Quadrant" solved it in an interesting way.
1. Calculate the altitude when the declination = 0 (i.e. at the equator)
tan h0 = cos A / tan lat
2. Calculate an auxilliary angle from:
sin x = cos h0 sin decl / sin lat
3. Calculate the altitude, h, from:
h = x + h0 if A < 90
h = x - h0 if A > 90
The value for h can then be used to find the hour angle.
There are some details to attend to if it is neccessary to deal with negative
altitudes, which are seldom a factor with sundials.
Best regards,
Jim
>
> From: Steve Lelievre <[EMAIL PROTECTED]>
> Date: 2005/03/27 Sun PM 03:29:54 EST
> To: [email protected]
> Subject: Azimuth to Hour Angle code wanted
>
> Hi,
>
> I'm looking for some BASIC code (or, second-best, C code) that calculates
> the hour angle from a known latitude, solar declination and azimuth.
>
> If anyone has a tried and tested code fragment to share, I'd love to receive
> it. You'll save me an hour or two, and a headache.
>
> Thanks,
>
> Steve
>
> ----
>
> Steve Lelievre
> Halifax, NS
> Canada
>
>
>
> -
>
James E. Morrison
[EMAIL PROTECTED]
Astrolabe web site at astrolabes.org
-
