# 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: Tony Oz
Date: 2020 Mar 9, 12:26 -0700

Dear Ed,

All those "on scale", "off scale" labels SHOULD be forgotten once for all - along with such remnants of the era of (algebraical) illiteracy as "same-name/contrary-name" mumbo-jumbo.

Just use the algebraic sign!

To obtain the value you should ALWAYS ADD (algebraically!) to the HS 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

This is the only way to do it properly.

The bonus is that with the same 360°mm' and 359°nn' values you can check yourself:

• calculate (360°mm' - 359°nn') / 4 and compare with the tabulated SDSun.

It is 2020 today, not ~400 years ago, when people debatted wheater the "0" is a number like all others or if it is valid to operate with negative numbers...

Warm regards,
Tony
60°N 30°E

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