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    Re: Swinging the Arc
    From: Frank Reed
    Date: 2012 Aug 19, 09:58 -0700

    Byron, you wrote:
    "Nothing on this earth is new. Every thing is a rediscovery"

    Of course, that isn't literally true --everything gets discovered once. But with a subject as well worked-over as celestial navigation, it sure seems that way! :) I have "re-discovered the wheel" quite a few times myself. :D

    The trouble with celestial navigation methodology today is that this subject has fallen into obsolescence, and this is where "entropy" takes over. So for example Bruce Bauer's claim, published in his very popular book back in the 1980s, that sights above 60 degrees (or so) are not worth taking because they are intrinsically inaccurate, is now frequently repeated as if "that's just the way it is" (it's not --they're only inaccurate if you swing the arc the "common" way). You'll also find that the amount of error that self-taught navigators consider acceptable is slowly rising (and has been doing so for decades) as newer navigators develop bad habits and innocently assume that the resulting errors are in the nature of the method itself. It's entropy. Soon I expect to hear "experts" saying that those "old time navigators" back in the 19th century must have had "amazing talents" or worse yet "a oneness with the natural order" that are beyond our modern science. That, too, is entropy.

    Your description of being able to keep stars in view is yet another reason to prefer what I have called the "preferred" method of swinging the arc. The "common" method has the stars zipping across the horizon glass and they can easily be lost. But in the preferred method, which again, was the standard method historically, a star remains more-or-less centered in the field of view, and it's much harder to lose.

    This business of swinging the arc in two different ways is an example of an insight that a navigation student can get from practicing lunars (or for that matter even horizontal angles between buoys). A sextant is an instrument that lets us look in two directions at once. They are not distinguished in any way. You can rock the sextant about EITHER direction. With lunars and with horizontal angles, it makes no difference since the objects being aligned are small in the field of view. Rock the Moon past the Sun or the Sun past the Moon. There's no distinction. But with standard altitudes, there IS a difference. The horizon, of course, is an extended object. Sweeping the Sun along the horizon by keeping the Sun centered in the field of view and rocking the instrument about the axis that points to the celestial body yields better results. And yet, the other "common" method necessarily works, too, as long as the object is not too high in the sky. As far as the sextant's optics are concerned, there is a complete symmetry between the two directions that the instrument "sees" superimposed in the field of view.

    -FER


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