[Radiance-general] Radiance sun position formula, solar position algorithm

[jeffn]

Hello,
I see that Radiance is using a short formula for solar position in sun.c.
 I checked the results provided by gensky for various locations and times
to the solar calculator provided by NOAA at
http://www.esrl.noaa.gov/gmd/grad/solcalc/.
I understand that most Radiance applications do not require an extremely
accurate position that more detailed formulas provide, but the difference I
find is many degrees (27 degrees for altitude at Cape Town) so I want to
confirm that this is working as expected.

******** Minneapolis
C:\Users\Jeff>gensky 01 29 15 +s -a 44.97 -o -93.25 -m -90
# gensky 01 29 15 +s -a 44.97 -o -93.25 -m -90
# Local solar time: 15.00
# Solar altitude and azimuth: 14.7 44.0
# Ground ambient level: 8.9

**NOAA altitude and aziumth: 18.29 28.58

******* Havana
C:\Users\Jeff>gensky 04 26 09 +s -a 23.133 -o -82.367 -m -75
# gensky 04 26 09 +s -a 23.133 -o -82.367 -m -75
# Local solar time: 9.53
# Solar altitude and azimuth: 53.5 -80.8
# Ground ambient level: 15.2

**NOAA altitude and aziumth: 40.22 -88.61

************ Cape Town
C:\Users\Jeff>gensky 08 14 13 +s -a -33.92 -o 18.37 -m 30
# gensky 08 14 13 +s -a -33.92 -o 18.37 -m 30
# Local solar time: 13.69
# Solar altitude and azimuth: 35.9 149.2
# Ground ambient level: 13.8

**NOAA altitude and aziumth: 41.882 177.13

************* Tokyo
C:\Users\Jeff>gensky 11 08 08 +s -a 35.7 -o 139.77 -m 135
# gensky 11 08 08 +s -a 35.7 -o 139.77 -m 135
# Local solar time: 7.95
# Solar altitude and azimuth: 12.2 -58.6
# Ground ambient level: 8.1

**NOAA altitude and aziumth: 18.84 -52.06

I also see that the Radiance code mentions Julian date but without a Julian
date formula - the input seems to be the Gregorian day and month. If I add
a Julian date conversion, the difference between the algorithms is even
greater.

Jeff Niemasz
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.radiance-online.org/pipermail/radiance-general/attachments/20130130/cf5c10e0/attachment.html>