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

[Greg Ward]

Given the tool construction, it would be simple enough to introduce a global variable that switches between the de facto solar angle calculation and a more precise version.  I'd like to hear from others that this is worthwhile before I spend too much time on it, though.  Also, whether it should be exposed as an option in all of the calling programs (i.e., IES calculation or more accurate on).

Is there a general consensus on which solar calculation is best?  I would just end up googling it, unless you or someone else has a strong recommendation.

Cheers,
-Greg

> From: Martin Gut <gut at Transsolar.com>
> Subject: Re: [Radiance-general] Calculation of solar declination in sun.c
> Date: December 17, 2015 8:55:38 AM PST
> 
> Hi Greg,
>  
> Thank You for the quick answer.  I don’t know about the history of this debate because I’m quite new in this mailing list.
> I think for the CIE standard stuff, sticking on the standard routines is ok.
>   But this formula is also used for all the dynamic daylighting stuff like
> Gendaymtx
> Gendaylit
> Daysim
> etc.
> also for shading studies: I’m wondering that so much computing effort is investigated to render the shading and on the other side the position of this shading is incorrect (especially in Autumn) because of
> a very rough calculation of the sun position. The error is not so big, but it could be avoided very easy by using a better formula for solar declination.
> Maybe there could be two functions, one for the standard calculations and one for “real”  calculations.
>  
> Martin
>  
>  
> Von: Greg Ward [mailto:gregoryjward at gmail.com] 
> Gesendet: Donnerstag, 17. Dezember 2015 17:14
> An: Radiance general discussion
> Betreff: Re: [Radiance-general] Calculation of solar declination in sun.c
>  
> 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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