My first question about solar declination was not answered, but I've fleshed out the problem some more and come up with a solution:

solar declination = 23.5*Sin(0.01721*d)

where d = 0 when the date is March 21st.

0.01721 is gotten by finding when 91.25*x is equal to [tex]\pi[/tex]/2. That also finds when sin(91.25*x) equals one which would be approximately during the summer solstice.

91.25 is simply 365/4 so that the seasons are even.

My equation is not exact, but is an approximation, which is not an issue for my purposes.

This equation seems to find the right answer within an acceptable range of error, but why is it that this equation is in wider use:

Declination is calculated with the following formula:

d = 23.45 * sin [360 / 365 * (284 + N)]

Where:

d = declination

N = day number, January 1 = day 1

from this website:

http://www.usc.edu/dept-00/dept/archite ... basic.html

That equation simply gives me the wrong answer.

Thanks for any explanation as to what I'm not understanding.