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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: Sextant index error and distance
From: Bill B
Date: 2014 Apr 07, 01:22 -0400

```On 4/6/14 9:51 PM, Debra Hillman wrote:
> ------------------------------------------------------------------------
>>Now what i need to know is,
> when you are looking at an object which isnt the horizon,example
> something closer like a rock ,a tree, i get a step when i look at them
> ,but my horizon is fine even after rocking .The arc is zero .Do i have
> more errors if this is happening? I would apreciate your views and help
> with this.Thanks.

Debra

I believe the challenge you mention above is called "parallax."

As an example, back in the day of the twin-lens reflex camera there was
a shooting lens, plus a viewing lens maybe 3 inches above the shooting
lens. Both lenses looked straight forward, their lines-of-sight parallel.

That works fine for a landscape or people perhaps 10 feet away. Now
imagine you want to take a photograph of the deadbolt lock on your door
from a foot away and frame the deadbolt on your ground glass. Later you
find your picture is mostly door and doorknob but no deadbolt. Why? The
two lenses are not looking at the same thing. That's parallax.

Now let's apply this to a sextant. When you are looking at the horizon
on the left side you are looking straight through the scope at the
horizon. The image on the right side is reflected to the mirror in front
of the scope from a mirror perhaps three inches higher than the scope's
line of sight. While that difference may not sound like much, with a
sextant you ideally wish to measure within a two tenths of a minute of
arc (12 arc seconds). 360 degrees in a circle, 60 minutes in a degree =
21,600 minutes. You want 0.2 of that so resolution of 108,000ths
of-a-circle .

Let's try simple trig. 3 inches at 10 feet = 1.5 degrees. 3 inches at 1
nautical mile = 8.5 seconds. (That's a lot of slop when your instrument
may be capable of measuring with 12-18 seconds of an arc or better.)
Calibrating with a natural horizon maybe 3 nautical miles away reduces
parallax error to 3 seconds of an arc (which is below the noise in the
system.)

If your sextant reads the same when aligning a distant sea horizon and a
rock ten feet away, then it is time to worry!

I would strongly suggest obtaining a copy of Bruce Bauers's "The Sextant
Handbook". It is not only useful aid for properly adjusting your
sextant, but offers many other useful tips.

Hope that helps. As to the sporadic shift key on your keyboard... :-)
```
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