# NavList:

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

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Re: Lunar Distances with Alex's SNO-T
From: Alexandre Eremenko
Date: 2006 Nov 1, 23:24 -0500

```
Bill,

> I recall Frank posting the method,
> but never quite grasped the principle.
> Why do we need 500 feet?

I wonder where Frank does it in Chicago:-)
I am thinking more of outdoor places,
like Celery Bog, etc.
500 feet we need because the angles involved are
very small.
Indeed, suppose your IC is 0.1' and you want to detect it.
the distance between the sextant mirrors is
about 2 inches. The two rays inclined at 0.1'
issued at a distance 2 inches will be still 2 inches
apart if you walk moderate distance from the sextant.
So you see nothing.
You have to walk far away for the rays to diverge by
2.1 inches, so that you can see the difference.
Simple computation shows that for this you have to
walk 260 feet away. But the difference between 2 inches
and 2.1 inches is quite small... So it is better to walk
twice that much.

Alex.

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