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    Re: A little cel nav in practice
    From: Frank Reed
    Date: 2022 Nov 28, 17:52 -0800

    Jeremy C, you wrote:
    "Interesting.  I wonder what nationality this mate is?  I haven't met anyone who does celestial navigation this way.  It doesn't appear to be either the "American" or "British" schools of navigation.  I wonder how accurate his final positions are."

    He's Georgian. Works for Delta Tankers. 

    And:
    "Observing a star azimuth is possible, although I'd tend to use the alidade on the repeater over a hand compass.  It just takes a bit of time to do so.  At low altitudes there obviously isn't much change in time, so the error is pretty minimal."

    Yes, and since he's doing his Hc calculation from a "best estimate" position, the intercepts should generally be quite short. If intercepts are a mile or two and assuming reasonable spread in azimuth for a pair of star sights, a +/-2° error in observed azimuth should only shift a fix by a couple of tenths of a mile and often less. This is also one of the arguments in favor of using various diagrams and lookup tables for azimuth when a DR position is used for the AP. All reasonable.

    Above, you wondered about the accuracy of his fixes. Yeah, I wonder as well. It's hard to say from the evidence I've seen, but I'm guessing we're in "plus or minus a mile" territory. And I can see issues in his methods that would lose a half mile here and a half mile there, but nothing huge.

    You added:
    "In my personal experience, I have found that stars under 10 degrees of elevation are generally un-reliable.  Likewise my experiments with moonlight sights away from twilight also yield questionable results."

    It's so hard to conduct experiments like this without a certain amount of confirmation bias though. As for me, I've found good results for altitudes down to about 2° altitude (below that the images dance and suffer from chromatic aberration --that "French flag" effect). But from 15° down to 2°, they're good as long as I'm careful to include the temperature/pressure factor. And you may have noticed that Davit (in that snippet I posted) included temp and pressure.

    I have almost no experience with moonlight sights on a sea horizon, but I have chatted with enough navigators over the years who use them regularly to be convinced that they work with some practice. I've also known a couple of navigators who have gone the "Moitessier" route and taken star sights on the ocean on moonless nights, but that's from a smaller vessel, completely "lights out" with an hour of full dark adaptation before a sight. That's something I would like to try. 

    Frank Reed

       
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