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    Re: A little cel nav in practice
    From: Frank Reed
    Date: 2022 Nov 29, 09:30 -0800

    Jeremy, you wrote:
    "Would height of eye affect the accuracy, viz a viz distance to the horizon?"

    A factor to consider is the temperature profile. Air temperature within the first hundred feet or so above the surface of the sea is supposed to be fairly consistent thanks to local surface winds, but is it? If you get your temperature and pressure from the observer's location, let's 85 feet high, does it match the numbers right at the water, which is what's required for the refraction correction? Of course one of the best reasons to skip altitudes below about 15° is so that we don't have to worry about T&P. And there's nothing wrong with that as a plan! :)

    And:
    "I see that his dip of -4' or so is consistent with the HoE of a Tanker bridge. "

    This is something that seemed a bit odd to me. Using his exact value of 4.5' for dip implies a height of eye of 21.5 feet using standard dip calculations or tables. Isn't that rather low for a big oil tanker? I don't know which vessel he was on but the "Delta Tankers" website lists Aframax, Suezmax, and VLCC tankers (that Canopus sight was taken on a voyage from Basra to Trieste, btw). They're all large. Even the very lowest accessible deck (with a reasonable range of azimuths for sights) on a fully-laden tanker would seem to be about 20 feet above the water. Add observer height and the bare minimum would be 25 feet. No? Aren't tanker bridge wings much higher than that? 

    "For the containership I'm on, my dip is usually closer to -9' which means a far more distant horizon."

    Twice as far, right? There's a possibility for different issues with anomalous dip at that range, but I don't see any reason that it would be a major factor, and of course anomalous dip would impact all celestial altitudes, high and low.

    "I won't speak for other navigators, who may certainly have better eyes and/or skills than I do, but I have made enough observations at low altitudes to know they don't work well for me from the ships I've sailed on."

    I know you've got detailed records, and I certainly recall your meticulous log of your sights from years ago when you came to Mystic Seaport (when was that anyway? ten years ago??)

    If you have time to experiment in the next few months (or years!), I would suggest trying some comparison sights: get a high altitude sight (30°-60° maybe?) and a low altitude sight (try for 5°-10°) on similar azimuths in the same twilight conditions. Maybe do a sequence: high, low, high, low. Then compare the intercepts relative to known position. Is there any sort of pattern that you can detect? Are your lower altitude sights consistently off in a systematic fashion, or do they show random error?

    A general note on potential accuracy and dip: 
    Whenever a confident celestial navigator (like the one I described in the original post) tells me that they can get fixes accurate to 0.1' on a regular basis because they "do the math right", I ask them about height of eye if I get a chance. How do they judge it? If the height of eye is wrong by a foot or two, how does it impact that fix? I can usually convince them that it's more to worry about than they may imagine... I know on the big ships that you captain, Jeremy, you can lock down height of eye quite well and your dip values are probably dead-on, but for celestial navigation on smaller vessels (even medium-sized commercial vessels), it's one of the critical limiting parameters in all of this.

    Frank Reed

       
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