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Author Topic: Calculating solar position  (Read 1077 times)

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Offline Dale Reid

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Calculating solar position
« on: February 18, 2017, 09:28:59 PM »
I've been interested in the solar position calculations for some time, specifically to help understand the calculation of the Maximum Solar Possible which we see graphed in WD.

The calculation of sunrise, hour angle 0, and sunset are obviously dependent upon the latitude, day of year, and time of day.

These values are alluded to if not spelled out in Jean Meeus book on Astronomical Algorithms.
 Meeus, J.
Astronomical Algorithms
. Second edition 1998, Willmann-
Bell, Inc., Richmond,
Virginia, USA.

However finding a relatively clear path to gather all those data and do the calculation has been less clear than I would have hoped for, even in the day of the internet and Google searches.

Recently with the help from a few generous folks on this forum, I've been stimulated to once again try to understand the mathematics behind the calculations.

Today I've found a couple calculators which basically do all one might ask for, and since they were sponsored by NOAA and NREL's (National Renewable Energy Lab) work, they have much of the C code along with extensive references on the calculations.

For those interested, the reference

http://www.nrel.gov/docs/fy08osti/34302.pdf

is a step by step summary.

For those who wish to see the results, a very nice calculator with entry for many items, some of which exceed the need for a casual observer (but perhaps of much more importance to the estimation of value of a solar installation for power generation), the visit and use the on line calculator at:

http://www.nrel.gov/midc/solpos/spa.html

This calculator states it is accurate in positoin to  +/- 0.003 degrees for the moon and +/- 0.0003 degrees for the sun, which is much more than my need is, but it gives interesting discussion of the factors affecting the apparent position.

Additional considerations are taken into account in another paper and calculator which deals with solar eclipses.

As always, it is amazing to see what our curious minds have been able to figure out and the depth of knowledge that has been shared to allow these calculations to be formulated.

I haven't stepped through all the information, and while I have grabbed the C source code, haven't walked through it yet, either.  Nonetheless, I hope to understand it well enough to use a less precise method to do rapid calculations in order to help plot the estimated Solar Max for my location, which was my original intent.





Offline mcrossley

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Re: Calculating solar position
« Reply #1 on: February 18, 2017, 11:34:36 PM »
I coded up all the Meeus calculations required for the 'approximate' calculations (good to a minute or better) for the Sun and Moon in Javascript for a couple of Microsoft desktop Gadgets I released back in the day, it was quite instructive.
Mark

Offline Dale Reid

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Re: Calculating solar position
« Reply #2 on: February 19, 2017, 04:05:44 AM »
Mark,
I know from earlier and more brain cells days, that reading about some thing in general was helpful, studying how it was done was another thing, then actually writing a functioning program was the ultimate learning experience.

Now I need to set aside the time to begin exploring this topic. 

Most folks won't give a rip.  Just like any other area of interest, some are more into it than others.

Great to know others have or had some interest and can recognize the initial hurdle that Meeus had to do for the initial work.

Then I see predictions where on earth and what time a moon-grazing star occultation will occur to cast a shadow on the earth and am amazed at the math and understanding and precision some people posses.

 

cumulus