# NavList:

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

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Sextant accuracy with short distance to horizon
From: Dan Allen
Date: 2001 Jun 21, 7:51 AM

```BACKGROUND
I was out sailing yesterday through the San Juan islands and in the Strait
of Juan de Fuca.  It was a gorgeous day aboard a friend's Nauticat 44 and we
took out an Astra IIIb sextant to work on sites from a rolling ship--the
swells at the east end of the Strait kept us rolling +/- 20 degrees either
way for about an hour.

Anyway, we wanted to measure our use of sextant accuracy so we did this: we
would record our exact GPS position, exact time, and the angle of our sun
shot and then calculate our position via an HP-48 program of mine to see how
close we came to the truth, the truth being approximated to within 12 feet
by the GPS position using a Garmin GPS III.

My friend hadn't used his sextant much before so his first shots were off by
300 miles, but quickly they got to within 5 miles of our actual position, a
great improvement.  I took one shot myself and got to within 4 miles.

Later we did the noon shot and his results were about 68 miles off for a
bunch of shots.  These were taken in Guemes channel with a local horizon
less than a mile away.

QUESTION
This led me to think more about possible inaccuracies of using a horizon
that is very close rather than naturally far away.  That is, typically you
use a horizon that is at the limit of your vision, perhaps 20 miles away.
Our local horizon was within a mile.  I don't remember reading much about
any errors with regard to this.  Do any on this list have anything to add
about possible errors of using a very near horizon?  I plan to go back and
reread the usual suspects on the topic, but I thought it may be of interest
to others.

Simply put, is there a formula or correction to give greater accuracy to
sextant shots taken using a very close horizon?  If one knows the exact
distance to the local horizon, can this be applied to the shot to give a
more accurate reading of the angle of a celestial body?

Or, is there a rule of thumb that says if the local horizon is closer than X
miles, one should use an artificial horizon instead due to big inaccuracies
that cannot be corrected for?

Dan

Daniel K. Allen
Navigate | Calculate | Sail

```
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