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    Re: Single-body fix (at mer-pass): some questions
    From: Frank Reed
    Date: 2019 May 8, 14:04 -0700

    Tony, you wrote: "And it is sort of language question again"

    Yes, I agree completely. 

    Some general comments on this method of getting a position from sights "around noon": you should expect in typical cases that the longitude you get from this process will be about five times less accurate than the latitude that emerges from it. In addition, like any running fix, it is dependent on knowing one's speed over the ground. If the navigator is affected by a significant N/S current, then the results will be worse. Note that we also need to correct for motion even when the observer is at rest. Ans that sure sounds contradictory! But, ok, it's not really correcting for motion anymore. There is an equivalent step that accounts for the Sun's changing declination even when the observer is motionless. Around the equinoxes, the change in Dec is equivalent to N/S motion at 1 knot. In the general case, we combine the vessel's motion with the Sun's motion. If I'm going nouth at 4 knots and the Sun is coming north at 1 knot, then we correct for motion by assuming a closing speed of 5 knots.

    By the way, the important point about culmination not matching peak altitude is that it throws off the time of culmination. The value of the altitude observed will be nearly the same except in unusually fast vessels. 

    Frank Reed

       
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