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

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Re: Beginner / Davis Plastic Sextants
From: Frank Reed CT
Date: 2005 Oct 6, 18:50 EDT

```Bill you wrote:
" The *really* nice  three-arm-protractors like Alex's Soviet unit can be
dialed in to 0.1' if  I
recall, so would demand input of equal precision and accuracy for optimal
results. "

An angle of 1 minute of arc is (and I do mean *IS*) a ratio of  1 to 3438
(10,800/pi to be exact). So an angle of 0.1 minutes of arc is a ratio  of 1 to
about 34,000. The distance "across" the object(s) and the distance "out"  to the
object(s) are in a ratio of 1 to 34,000 when the angle is 0.1 minutes of
arc.  Given that, is there any chance that this tiny angle could be  meaningful
in plotting with a three-arm protractor?? Assuming that the chart  itself is,
let's say, one meter across, if I extend a line all the way across  the chart
and shift the end-point by 1 millimeter, I'm making an angular shift  of only
1/1000 or in other words about 3 minutes of arc. No horizontal angle (as
between buoys) would ever need to be measured to an accuracy better than this  for

-FER
42.0N 87.7W, or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

```
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