How to calculate sun position?

[Trond Hoyem]

This function should work, though it was not tested much:

Code:

-- solar altitude, azimuth (degrees)
function sunposition(latitude, longitude, time)
 time = time or os.time()
 if type(time) == 'table' then
   time = os.time(time)
 end

 local date = os.date('*t', time)
 local timezone = (os.time(date) - os.time(os.date('!*t', time))) / 3600

 if date.isdst then
   timezone = timezone + 1
 end

 local utcdate = os.date('*t', time - timezone * 3600)
 local latrad = math.rad(latitude)
 local fd = (utcdate.hour + utcdate.min / 60 + utcdate.sec / 3600) / 24
 local g = (2 * math.pi / 365.25) * (utcdate.yday + fd)
 local d = math.rad(0.396372 - 22.91327 * math.cos(g) + 4.02543 * math.sin(g) - 0.387205 * math.cos(2 * g)
         + 0.051967 * math.sin(2 * g) - 0.154527 * math.cos(3 * g) + 0.084798 * math.sin(3 * g))
 local t = math.rad(0.004297 + 0.107029 * math.cos(g) - 1.837877 * math.sin(g)
         - 0.837378 * math.cos(2 * g) - 2.340475 * math.sin(2 * g))
 local sha = 2 * math.pi * (fd - 0.5) + t + math.rad(longitude)
 local sza = math.acos(math.sin(latrad) * math.sin(d) + math.cos(latrad) * math.cos(d) * math.cos(sha))
 local saa = math.acos((math.sin(d) - math.sin(latrad) * math.cos(sza)) / (math.cos(latrad) * math.sin(sza)))

 return 90 - math.deg(sza), math.deg(saa)
end

Hi

I used this today. What I see is that when the azimuth get to 180, it starts counting down rather than going on to the full 360... Any ideas why?