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    Re: Solstice
    From: Frank Reed
    Date: 2021 Jun 22, 23:02 -0700

    David C, you wrote:
    "Frank you have raised a subject that I find confusing. I calculated  gha sun - gha aries for the time you specified. The answer was 90° (give or take a few tenths of minute of arc beeause I did not pay attention to interpolation). Is this the SHA sun - your figure was 270?"

    Did your result perhaps come out to -90
    ° (which is, of course, identical to 270°)? I think that explains the apparent discrepancy. You can check on these numbers and experiment a bit using one of my web apps. Go here (it's preset to your approximate location, near as I can remember, which impacts some results):
      Nautical Almanac data web app.
    Click details in that app, too.

    You added:
    "Many of my navigation text books have lengthy chapters on time which I find confusing."

    I agree that they're confusing because they're confused. Most of them are awful. Much of this is because global time and the transition from one longitude to another was still a sort of 'rocket science' for navigators in the middle of the last century. But today even children get it. They know because they have experienced directly, or almost directly, how time varies around the globe yet stays the same. We can speak to people in other parts of the world (you and I and others have done this ourselves during my "Office Hours" gatherings) and experience different times of day at the same absolute time. So much of the weird book-keeping of the mid-20th century now seems superfluous. Time is not difficult!

    You added:
    "The texts are mostly based on the RA and R, E almanacs rather than the GHA version used from the 1940s."

    And bear in mind also that many people who feel they know quite a lot about navigation will have no idea what you mean by "R, E almanacs". That was a British thing. There never was an "R and E" period in the American nautical almanacs. There was, however, an "RA" period. Modern navigators accustomed to GHA and SHA are often befuddled by the astronomers and their "RA". But it's not comlicated at all if we remember that RA and SHA are fundamentally the same thing. SHA and RA are longitude-like coordinates of stars, planets, and other celestial bodies "on" the celestial sphere measured relative to a "fixed" point in the heavens, which of course we know as "Aries" or sometimes the "First Point of Aries". That's the sky prime meridian. RA differs from SHA in one minor geometric fashion and sometimes in a minor "units" fashion. Geometrically RA=360°-SHA and equivalently SHA=360°-RA. This means nothing more than measuring one coordinate clockwise around the polar axis from Aries while the other goes counter-clockwise. Also SHA is universally listed in degrees while RA is normally, but by no means always, given in hours and minutes at the usual translation rate of 15° per hour.

    You ended by asking:
    "Can you please  give an explanation of SHA sun and the solstice based on the GHA almanac."

    What I wrote above should be preamble for that, so if it's still not clear, please elaborate on your question. Or forget the question, and we can just go on from here with other aspects of the concepts. :)

    Frank Reed

       
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