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    Re: Sextant problem
    From: Frank Reed
    Date: 2025 Jan 21, 06:47 -0800

    Peter:

    Thank you very much for including the photos of your index error test process. It seems likely now that you have fallen into a trap. The Sun-Sun method can be used in a couple of different ways. You can place the Sun's images directly on top of each other, superimposed. This is a viable test for index error not really different from the usual horizon (or distant hills) test. For this test, you can use anything further away than about one mile. The sea horizon is always further than that for any realistic observer height of eye (any height greater than one foot). Another option, which I recommend is to use aircraft contrails. Contrails only appear at high altitudes, typically several miles, with common cruising altitudes around six miles high, and if you orient a fresh contrail with sharp edges across the sextant's field of view, you can use it just like the horizon.

    In the standard Sun-Sun index error test, not as above, the Sun images are placed offset from each other, just touching, limb to limb. It seems this is what you were trying to do, but you didn't have the full instructions. I recommend you ignore the Sun-Sun test for now. You don't need it. It appears that you have read a partial description of the technique.

    To reiterate from my previous message, in order to do this "right", as it was originally described and intended, you place one Sun image above the other ...just touching, limb to limb, like in your photo. This is not the point of zero index error. Your sextant should read an amount of approximately 32 minutes of arc, on or off the arc when aligned like this. You record that value, and then re-adjust so that the two Sun images are swapped. If the reflected Sun was previously on top, you turn the micrometer until it is below the direct image of the Sun, just touching as before. You read the micrometer again. You should once again find a value near 32 minutes of arc, but if the previous reading was "on arc" (meaning in positive territory on the angle scale), then this reading will be "off arc" (meaning in negative, "below zero" range on the angle scale). Of course, your two values won't be perfectly and exactly matched. You might get 32.1' and 31.7'. Subtract one from the other and divide by two, and that difference is your index error. But as I say, you don't need this additional test. Just use the standard horizon technique, and you're all set.

    Frank Reed

       
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