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    Re: International Date Line
    From: David Pike
    Date: 2021 Dec 5, 08:40 -0800

    Philippe Posth you asked: I'm wondering about the date line, in the middle of the Pacific Ocean. How do you take into account the passage of this line in astronomical navigation? In the Nautical Almanac, should you skip one day, use the same twice, or do nothing ...? Thank you for your experiences.


    By the time I’d written this, two others had answered, but I’ll post anyway; it give the same answer with a different slant.  The Nautical Almanac provides the GHA and declination of the Sun, Moon, and planets, and GHA Aries for a particular time and date at Greenwich.  So long as you know that, it’s all you need.  To get LHA at your longitude, just look up GHA for that Greenwich date and time and subtract your longitude west.  If the result comes to less than zero add 360 degrees.  Alternatively, add your longitude east.  If the result comes to more than 360 degrees, just subtract 360.

    What about declination?  Declination of the Sun, Moon, and planets changes with time like GHA, but the declination of a celestial body is relative to the Celestial Equator, not your longitude, so you can use the straight Nautical Almanac values from the daily Greenwich page (adjusted for the exact Greenwich time of course).   

    Incidentally, when the Vulcans, which had a GPI 6 that you couldn’t just flick the counters from W to E on, reached 180W, resetting the GPI to 180E would have taken hours, so they simply took a second GPI 6 already set up to 180E.  As they approached 180W, they simply DR'd ahead a few minutes across 180 W/E, pulled out the GPI reading 180W, pushed in the one reading 180E, and reset it to their DR position which took no time at all.  DaveP

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