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    Re: Sextant index error and distance
    From: Frank Reed
    Date: 2014 Apr 7, 13:14 -0700

    Debra, you wrote:
    "when you are looking at an object which isnt the horizon,example something closer like a rock ,a tree, i get a step when i look at them"

    Peter and Bill have already answered this very clearly, but I'll just offer another spin on it. To check index correction, you have to use something that's a few miles away. The sextant has a sort of "stereoscopic" view of the world with one "eye" (the index mirror) about two or three inches above the other "eye" (the horizon glass). In celestial navigation we're always looking at objects that are at very great, astronomical distances, so to test index error we also have to look at something reasonably far away. A couple of miles is a reasonable distance for most work, and conveniently, the sea horizon is some miles away from a typical boat and it's always available whenever sights are practical so that's the natural choice. If you look at something that's too close when checking index error, your error in minutes of arc will be given by 3438*H/D where H is the "height" between the two lines of sight of the sextant (two or three inches as a I mentioned) and D is the distance to the target in the same units. For example, suppose you try to use a building roof line that is 100 feet away as a substitute for a horizon. And suppose H is 3 inches. The ratio H/D is 3/1200 or 1/400, and then multiplying by 3438 we get 8.6' of arc. This would be the erroneous apparent index correction that you would get from using such a close target. This is not an additional error; it's just a misleading procedure. If the target is ten times further away, the error is ten times smaller. And if the target is 100 times further away (10,000 feet, near two miles), then the error in the index correction would be 100 times smaller which is less than a tenth of a minute of arc and thus nothing to worry about (unless you're trying for really high accuracy, "chasing tenths" as Bill Morris has put it, and then you want to go for a target that's five or ten miles away).


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