[Radiance-general] Calculation of solar declination in sun.c

[Greg Ward]

Hi Martin,

This debate comes up every so often with the solar calculations used in Radiance and related tools.  The main question is, do we stick with the established standard, which provides for easy "apples to apples" comparisons, or do we update our formulas to get a more precise answer?  In the case of the original CIE sky models, which this solar calculation is used in primarily, the accuracy is not that great, so getting the sun in exactly the right place is a minor quibble.  If, on the other hand, you need precise sun position for solar shading studies or the like, then it's easy to argue for a better formula.  I don't think computing power was ever the issue with Radiance.  We don't compute the sun position for every ray-traced or anything silly like that.

As Rick points out, the use of 368 is part of the IES standard calculation, but I'm not entirely sure what anomaly it is correcting for.

Cheers,
-Greg

> From: Richard Mistrick <RGMARC at engr.psu.edu>
> Subject: Re: [Radiance-general] Calculation of solar declination in sun.c
> Date: December 17, 2015 7:18:53 AM PST
> 
> I’m not certain why it is 368, but this equation has been in this format in the IES Lighting Handbook for many years.
>  
> Rick
>  
> From: Martin Gut [mailto:gut at Transsolar.com] 
> Sent: Thursday, December 17, 2015 6:05 AM 
>  
> Dear Radiance Experts,
>  
> I have two questions regarding the calculation of solar declination:
>  
> 1.       Why has the year in function sdec  in  file sun.c  368 days instead of 365 as in the original formula from Cooper ?
>  
> return( 0.4093 * sin( (2*PI/368) * (jd - 81) ) )
>  
> https://github.com/NREL/Radiance/blob/master/src/gen/sun.c
>  
> 2.       Why does Radiance not use a better formula, which takes into account, that the Earth orbit is not a circle?
> With today computing power, there is no more reason to use this simple formula
>  
> Thanks in advance
>  
> Martin
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