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

[Greg Ward]

Hi Jeff,

The NOAA calculator takes degrees east longitude, whereas gensky expects degrees WEST longitude.  (Same for the standard meridian.)

Try it; you'll find much better agreement.  (I get within half a degree for Cape Town using your parameters.)

It's confusing all the different standards, isn't it?

Cheers,
-Greg

> From: jeffn <jeffn6782 at gmail.com>
> Date: January 30, 2013 2:08:53 PM PST
> 
> 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
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