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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Index Error
From: Fred Hebard
Date: 2004 May 7, 01:00 -0400

```Robert,

I usually take three replicates of this shot when setting the index
error.

When I first started doing this, I also got sizes smaller than the sun,
as you do.  I then readjusted what I took to be tangency between the
two images and got better results, although now (the past 16 months) I
am frequently high by about 0.05' to 0.1' on the SD of the sun.
However, if you are shooting the sun using a reflecting artificial
horizon, this does not jibe with your sight error of less than 0.1'.
You should have been off by the amount you are off measuring the SD,
adjusting for the difference in calculation, which might halve or
double your error, as appropriate.

Fred

On May 5, 2004, at 4:57 AM, Robert Eno wrote:

> 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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>

```
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