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Good point but this was taken into account. For example, yesterday, I
came home for lunch and couldn't resist taking a quick noon sun shot.

Index error calculations were as follows:

32.'0 on  30.'6 off
Difference = 1.4
1.4/2 = 0.7
IC = -0.7

Now let's take the next confirmatory step:

 32.'0 on + 30.'6 off = 62.6

 62.6 - 0.7 = 61.9

61.9/4 = 15.5 (rounded up)

Actual semi diameter = 15.9
add'l error = 0.4'

In calculating my noon latitude, I used an IC of -0.7', ignoring the
other mystery error. My noon latitude differed from actual by 0.1'.
Pretty close and one could say: "fluke", but I have been getting this
kind of consistent accuracy with my sextant.

So the question is, where in the hell does this "other" error come from
and if I wanted to factor it in to my IC, how do I go about doing this?

Theoretically, the sum of my off and on readings should have been 63.6
but I got 62.6.  I should know all of this by now but even someone with
experience needs to go back to the fundamentals every once in a while.

I am baffled.  Additional comments welcome.



>According to the 2004 nautical almanac the semi-diameter of the sun
varies between 16.3' at perihelion to 15.8' at aphelion. Or the diameter
varies between 31.6' and 32.6', for a difference of 1.0' or 2 X 0.5'
>
>Could this be the source of the error?

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