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

[Nathaniel Jones]

I'll echo Chris's comment, and note that NREL has been nice enough to post
their c source code online (for non-commercial use only). They claim
accuracy within 0.0003 degrees for the years -2000 to 6000.
https://www.nrel.gov/midc/solpos/

Nathaniel

On Thu, Dec 17, 2015 at 12:48 PM, Christian Humann <
chris at christianhumann.com> wrote:

> Hi Greg,
>
> I’ve found the NREL algorithms to be quite accurate:
> http://www.nrel.gov/docs/fy08osti/34302.pdf
>
> Chris
>
>
>
> On Dec 17, 2015, at 9:21 AM, Greg Ward <gregoryjward at gmail.com> wrote:
>
> 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 <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 <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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