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    Re: Calculating Twilight
    From: Frank Reed
    Date: 2014 Sep 20, 10:55 -0700

    Bruce, you wrote:
    "I could preset the angle, but that is not a “fair” procedure."

    You should preset the angle. It's a fair procedure, in the sense of corresponding to reasonable sea practice. A navigator is never lost. You always have some rough idea of your position unless you've been thrown into some new navigating situation --when would that happen in the modern world? ...maybe a reality tv show dumps you somewhere in mid-ocean?? And even then, you're only really "lost" for a couple of hours if you can see the stars. As soon as you have a rough latitude, presetting the sextant is 100% legitimate and "fair".

    I agree, though, that latitude by Polaris is much more difficult than you might expect. It requires a crisp horizon (yes, more difficult near shore), careful timing, and also it helps to be properly dark-adapted. In morning twilight, if you avoid lights, you'll be able to see the horizon earlier than normal. In evening twilight, this is harder to arrange since it takes about fifteen minutes to become properly dark-adapted. That fifteen minute period happens to be roughly the pace of the transition from light to dark in twilight, so in evening twilight, by the time you're dark-adapted it's probably too late (I suppose a case might even be made that human vision did not evolve with faster dark adaptation because the speed of twilight is the main selective factor).



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