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    Re: Lunar Calculation Presentation
    From: Brian Walton
    Date: 2022 Sep 19, 22:13 -0700

    I have just returned from an inter-continental sailing trip during which I attempted a "Chichester" time check. Poor wifi, and events in UK yesterday delayed this post.

    2 days after this last full moon I did morning stars, with a moon shot. The PLs crossed as well as one can expect on a small bumping boat, so my timepiece was correct. Part 1 of the check.

    I went about replotting the fix deliberately using the wrong time, to see if a split in westing between the moon and stars could determine if the watch was fast or slow. The moon was 14 away on 244°, a 10 mile westing. I reasoned that if the split was 10 mls, that is 40" time at the equator, 80" at 60°N, so 50" at 36°N. The moon was too low, too far advanced, so ahead. Watch slow. One of the stars was Diphda, deliberately chosen because it was on roughly the same azimuth as the moon, as advised by Frank Reed. Over a 25" period Diphda's Hs reduction was 1.2 less than the moon's. Looking good.

    I was using a 140 year old box sextant with vernier, readable only to 1 minute, and plotting on a self made exercise- book page. I consider a 5 mile error good. Normal for me, but perhaps not good enough for this.

    Whilst pondering whether to advance the star DR by 10 miles west, and recalculate the moon 50" later, other duties called, and I popped my head out to see the Pillars of Hercules. We continued on our trip from Ceuta to Sotogrande.

    If my logic was right, the new plot might have resulted in a better cross. If it made a bigger split, I could have reversed the procedure. Work in progress.

       
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