# 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, 09:45 -0500

```
Dear Frank,

> If you have a large enough work area (a couple of
> hundred feet) and a decent laser, it should be infallible.

Unfortunately, I do not have large work area.

> Actually, the correct horizon SD
> at the time of the observation was 15.9
> (calculated from 27.27% of the HP --you should
> always use a calculated SD when
> doing lunars). Then you need to augment it for
> altitude which gives 16.0
> minutes  of arc. The result is then quite a bit closer, but yes, a little off --an
> error  of 0.14 minutes of arc in the Moon's diameter.

Can you just write the formula?

> Based on your  similar observations you wrote:
> "Now I derive from my data IC=0.7 which also seems absurd."
> I checked my notes, and that was the IC of that sextant
> last time I  checked.

But this implies that all my observations
with this sextant are very much off.
(Assuming that it always was 0.7. I adjusted it only
once in the very beginning, removed the side error.)
Including all test of the index from stars.

Returing to my sextant,
years I own it.

If it really was closer to -0.7, this would explain
most of my overshots with star distances.
But then how could I be so badly wrong with IC all the time?
Including Sun and stars.

The reason why Moon test can be unrelibale
is that we can never be sure that
we use the edge of the Moon,
rather than the edge of the shade.

On the other hand, I realize that in the two examples
we discuss, we both had overshots in 4SD,
and confusing the edge with the edge of the shade
would give undershot.

All this seems very strange.

> This is one of the simplest observations you can make
> with a sextant. If
> it doesn't work, what on Earth could explain the error??

Could you make Sun and Moon tests with my sextant
on the same hour and compare?
Or maybe a laser test?

Alex.

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