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

[Humann Chris]

Hi Jeff,

Part of the discrepancy you're seeing is from using negative values  
for the longitude and meridian settings in Gensky which wants positive  
values for western longitudes.
Repeating your gensky command using positive values:
gensky 01 29 15 +s -a 44.97 -o 93.25 -m 90

I get:
Solar altitude and azimuth: 17.8 38.4

Also I think you may have written down the NOAA azimuth result  
incorrectly for Minneapolis. Rather than 28.58 I think it should read  
218.58 (which is around what I got at the same site). When you  
subtract 180 degrees from 218.58 you get 38.58.

Chris

Christian Humann ~ Associate
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On Jan 30, 2013, at 2:08 PM, jeffn wrote:

> 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 _______________________________________________
> Radiance-general mailing list
> Radiance-general at radiance-online.org
> http://www.radiance-online.org/mailman/listinfo/radiance-general

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