# NavList:

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

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Re: sextant precision.
From: Bill B
Date: 2005 Jun 23, 20:12 -0500

```> So at the moment, my muse, the "should" does, and the "shouldn't" does (to a
> magnitude related to the "should" yet be be determined).  Theory will follow
> experiments with greater controls in place.

Frank

More carefully controlled observations indicated the following:

Given: Angle of glass relative to light rays is held constant.

Given: Thickness of glass and refraction index is held constant.

If the object remains at a constant distance from the glass, the
displacement will decrease as the eye is brought closer to the glass.

If the eye distance from the glass remains constant, the displacement will
decrease as the object is moved away from the glass.

My theory?  Given a great distance to the subject and a relatively thin
shade close to the mirror/observer, refraction from a slightly off-kilter
shade is insignificant. My other theory is that the ratios, mathematics and
formulas are best left to those trained in mathematics and physics ;-)

While we are on the topic of refraction, a while back you mentioned:

"There's more to it than that.  You can get more complicated layering of air
layers with different temperatures close to shore. Near Mystic, Connecticut
in the spring and fall, it's not that  uncommon to see five or more
arcminutes of anomalous dip, and this is for sights  taken on water (but
within sight of land)."

I queried:

"This is something that has been bothering me since my experiences on Lake
Michigan last weekend.  With approx. 65F water temperature, 85F air
temperature, and a white strip apparently covering the sharp horizon, I
would have expected my Ho's to be larger than my Hc's as the line I used
between the water/white strip might have been lower than the actual horizon.
I was surprised to find all my shot Ho's to be smaller than Hc's.  I would
not discounting pilot error on my part of course.

As a rule of thumb, what is the trend of,  "...it's not that  uncommon to
see five or more arcminutes "of anomalous dip, and this is for sights  taken
on water (but within sight of land)."  That is, how will it shift Ho vs. Hc?
Is there a semi-predictable trend for cold water/warm air and/or warm
water/cold air?"

"I'd like to pose a question to the experienced seafarers and navigators on
this list.

What atmospheric and weather conditions would you consider to be ideal for
taking sextant observations at sea? Why?

What atmospheric and weather conditions would you consider to yield
observations of questionable value? Why?

I am excluding sea state here, but for argument's sake, let us assume that
the seas are flat calm.

I have few questions to add to this, but I want to wait and see what kind of
anwsers come across to the first question.  I can explain the reasons for my
questions as the discusssion (hopefully) moves along."

I would be grateful if you and others would address these question, now that
there are opportunities/weather to do observations on the water instead of
in the yard.

Thanks

Bill

```
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