# NavList:

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

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Re: Star-star distances for arc error
From: Bill B
Date: 2009 Jun 24, 04:56 -0400

```Douglas wrote:

> I have tried it and I don't rate it as practical...

I have no intentions of being argumentative here, but rather of repeating
the old saw, "Don't blame poor craftsmanship on poor tools.  If you doubt
that, try placing a fine tool in the hands of a poor craftsman."

Using star-to-star distances to measure accuracy along the arc is not easy,
but can be precise.

There are two challenges. The first is predicting the star-to-star
distances. An internationally known math professor and contributor to the
list (Alex) and I compared our spreadsheets (mine with a refraction formula
provided by Alex from a text that calculated theoretical to observed as
opposed to flipping the almanac observed to theoretical), plus corrections
for altitude, pressure and temperature.  In all cases we were within 0.1
minute of each other once we identified the glitches in our spreadsheets.
So much for prediction.

The second challenge is observation.  This requires a good bit of practice
and there are many tricks/techniques to be learned and practiced.  Alex and
I spent many hours with at least two sextants (usually a Sno-T and and Astra
IIIB) with our usual three scopes--and for a period of time with five
scopes, two of which we were evaluating for a manufacturer.

I can tell you that no matter what the combination of scopes and sextants,
our observations agreed within 0.1 to 0.2 minutes of an arc for any star
pair, and we did find a strange spot on my sextant around the 90d mark that
was a full minute off.

IMHO there are personal errors, there are ghosts in the machine that are not
detected by bench tests or recorded on certificates; but if you cannot sit
down with a good sextant and artificial horizon at a known location on land
and (with averaging) come within 0.3 minutes of your location with an LOP
the problem is most likely not with the instrument, but rather with your
technique and field calibration. I do not accept 1.5 minutes error in a
modern sextant as reasonable.

Again my opinion, but if you gain a "mastery" on land or shore you can
eliminate a lot of slop on the water, and focus on the variables that cause
you pain afloat.

Bill B.

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