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    Re: Determination of the GHA of Aries
    From: Sean C
    Date: 2020 Sep 11, 00:18 -0700


    You wrote:

    "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."

    Actually, it does ... but it's not obvious at first glance. Under the "Calculate" menu and the "Date and Time" sub-menu, there is a selection called "Sidereal Time". Selecting that will give you the mean and apparent sidereal times for Greenwich and whatever location you have set in the location manager. The one you are interested in is "Greenwich Apparent Sidereal Time" or "GAST".

    GAST is GHA Aries. It's just given in hours, minutes and seconds instead of degrees and decimal arc minutes. All you have to do is multiply GAST by 15 to get GHA Aries in degrees.

    Of course, there is also the free Excel almanac I made, available here. ;) No need to do any math - it always shows GHA Aries. But I think David Fleming is right: you don't really need GHA Aries for this problem. There are some programs available online made specifically for installing solar panels. But any almanac (including mine) should give you azimuth and altitude, from which you can calculate shadow length and direction.


    Sean C.

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