# NavList:

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

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Re: Sextant Scope Parallelism (was Re : SNO-T Sextant)
From: Frank Reed CT
Date: 2004 Aug 12, 23:31 EDT
Robert G, you wrote:
"That’s where you got me. If the sextant is vertical you are measuring the
correct angle. If the scope is pointing off center by a small amount then
you see the sun a little displaced from the center of the scope. But isn’t
the sun still in the center of the horizon mirror and therefore in the right
place for the measurement.  If you insist on keeping the sun in the center
of the scope I don’t think that you can get the sun to the bottom of the
arc, it will wander off center as you get to the bottom.  Can you force it
both to the center of the scope and the bottom of the arc at the same time
if the scope is out of Parallelism?"

The purpose of "swinging" the sextant is to ensure that you are holding it vertically. So let's set that aside. Let's suppose you have ensured that the sextant is exactly and precisely vertical by swinging or even by mounting the instrument on a pole. With that done, you can still rotate the sextant about its vertical axis so that the Sun and the spot on the horizon beneath will pan from left to right across the field of view of the telescope. If the telescope is not parallel to the sextant frame, the contact between horizon and Sun will change as you pan, typically only by a few tenths of a minute of arc. In practice, you would almost never notice this effect with a standard Sun sight, except at noon, because the Sun is moving so fast. The best way to test for this alignment issue is by bringing two star images together. Observed angles between stars do change (because of changing refraction as the stars travel across the sky), but the change occurs very slowly and would not be obervable for several minutes. So you bring two stars together in the field of view of your sextant. Usually you will do this somewhere near the center of the field of view. Now rock the sextant back and forth so that the superimposed star images shift from left to right. If they do not remain superimposed across the whole field of view, the telescope is not parallel to the frame. When shooting lunars, where a couple of tenths of a minute of arc can be a big deal, this alignment error can lead to surprising errors. Speaking from experience, getting the scope parallel goes a long way towards making the sights more accurate.

Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois

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