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    A little cel nav in practice
    From: Frank Reed
    Date: 2022 Nov 27, 09:28 -0800

    I was chatting yesterday with a navigator who is a mate aboard a Greek-owned large oil tanker. He does celestial sights regularly, and his procedures are interesting.
    Some details I noticed:

    • stars sights in the middle of the night when the Moon is out.
    • Hc values calculated by trig tables (paper!) and the standard "cosine" law.
    • values from the trig tables are five-digit precision
      but he works his products and final value for sin(Hc) to ten digits since he believes (erroneously) that this is best for accuracy.
    • [see image] sometimes lists cos(LHA) as sin(LHA) --which is incorrect formally-- but that's how it's found in the tables he's using.
    • low altitude stars preferred because he observes, rather than calculates, azimuths of stars
      and he notes that he does this with a handheld compass compared with a nearby repeater of the vessel's primary compass.
    • each star sight is worked from a DR-updated AP and then plotted from the final AP --that's actually easier than advancing LOPs by plotting,
      but the APs are adjusted only to the nearest mile, which makes the tenth of a minute calculation precision somewhat superfluous.

    The images below show his working of a sight of the star Canopus about 60 nautical miles northeast of Bahrain. The only missing detail in the setup is height of eye. He uses -4.5' for dip later on the page. I take note of some issues and problems above, but I don't want to suggest that he's "screwing up". Celestial navigation is "cultural". Navigators develop habits and adopt lore and learn from other people with habits and their own lore. We see these personal rules in primary source evidence. Have to look over their shoulders to see how real navigators work.

    Frank Reed



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