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Re: Determination of the GHA of Aries
From: Brian Jones
Date: 2020 Sep 10, 02:54 -0700

Hello Dave,

Thank you very much for giving me some insight into my Aries problem. This all began when I decided to put a solar system on a barn that is off-grid. Due to topology and vegetation it was guesswork as to the best tilt angle and azimuth to maximize available insolation—a shadow problem; so I decided to write a program to compute the shadow lines over time. As the Air Almanac was available in PDF format I could download it and write a parsing program to extract the Sun’s GHA/dec for the current year and put it into a table I could use programmatically, but then USNO shut down their web site and I realized that I could not rely on that source so it became necessary to learn how to compute these values without the aid of an almanac, a challenge as my only background in this is from my time as an Air Force navigator—pre GPS—relying on printed tables without the knowledge of how they were created.

I purchased the U.S. Naval Observatory’s MICA program from Willmann-Bell, LLC (www.willbell.com) that provided (in a rather convoluted way) the GHA/dec for the Sun for a selected year (1800-2050), but it does not include the GHA of Aries. Therefore, from my solar tracking program’s point of view the Sun’s GHA/dec is the only data it has to determine GHA/Aires

I’m glad to learn that GHA/Aries with respect to UT is linear, making it much easier to work with. I was trying to determine a way to compute GHA/Aries from GHA/Sun compared to the first point of Aries without much luck. I’m now wondering that since GHA/Aries is linear but GHA/Sun is not, that a correction function is necessary to adjust the GHA/Sun to make it appear as though it is not effected by EOT before using it to determine GHA/Aries.

Am I going down a rabbit hole?

Thanks, Brian

I probably should have stated this up front, the objective is to create a data table with an record for each minute of the year, therefore, the date/time is assumed. Example: Record 1 is Sun/Aries data for 01Jan2020/0000 hours, Record 2 for 01Jan2020/0001 hrs...31Dec2020/2359 hrs.

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