Next message: Rodney Myrvaagnes: "Re: Sextant optics"
Question for the list on some fundamentals and I add that I am somewhat embarrassed to ask such a question.
A method I have long used for determining index error when taking practice observations on land is to use the sun's limbs by setting the sextant at 32' on arc, touching up the reflected and actual sun's limbs, recording the number, then doing the same off arc. One half the difference between the value on and off arc should give you your index error; the sign being the greater of the two values. I have found this to be a reasonably accurate method for determining index error; as long as the sun is at least 30 or more degrees above the horizon.
Here's the problem: theoretically, if one adds the reading off and on arc, subtracts the index error and divides by four, then the result should equal the tabulated value for semi diameter in the Nautical Almanac.
I have seldom found this to be true. The result often differs from the tabulated value by up to 0.4' of arc. That's a lot.
Does anyone have any explanation for why this might be? I should add that my sextant, a Plath, has never suffered any grievous insult so I do not believe that instrument error is the cause. Hundreds of observations also indicate that the instrument is pretty darn precise.
