# NavList:

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

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Re: Using the sun's direct and reflected image to determine a sextant's IE
From: Paul Hirose
Date: 2020 Mar 9, 23:03 -0700

```On 2020-03-09 13:16, Tony Oz wrote:
> To obtain the value you should ALWAYS ADD (*algebraically!*) to the H_S do:
>    -- measure the Sun's width by touching the direct and reflected images
right-to-left (or upper-to-lower if you want) and then the left-to-right
>    -- imagine that the zero on the arc is marked as 360° and therefore if
the index is to the left of it - read it as 360°mm', if the index is to the
right - read it as 359°nn'
>    -- calculate 360° - (360°mm' + 359°nn') / 2

The calculation is simplified if you imagine the -1° mark on the limb is
0°. For example, if the sextant reads 1′ high and semidiameter = 15′,
the limbs touch at 1°31′ and 0°31′.

Index correction = 1° - (1°31′ + 0°31′) / 2
= 1° - 2°02′ / 2
= 1° - 1°01′
= -1′

If it seems more natural, you can reverse the operands in the
subtraction. Then the result is index error, which is *subtracted* to
get the corrected angle. In fact that's the method I prefer.

I also adjust my sextant so an index error determination always produces
a small positive result. I believe there's more chance of a blunder if
IE is sometimes positive, sometimes negative.

There's much confusion about the sense of IC vs. IE. A few years ago I
tried to correct someone on this. He replied that many other people do
the same thing, and seemed content with his understanding of the matter.
```
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