# NavList:

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

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Re: Bowditch tables and sexant parallax
From: Frank Reed CT
Date: 2005 Apr 28, 17:55 EDT

```Bill wrote:
"How the heck do I correct for  parallax error for two points at varying
distance? "

I don't think anyone  bothers with this. It might be an interesting
mathematical puzzle, but I don't  think it would ever matter in practice.

And:
"and a three-arm  protractor is precise to 0.1'."

But is it that accurate? Let's  suppose the arms are 50 cm long. Placing the
end points at 0.1 arcminute  accuracy would be equivalent to placing the tips
of the arms with a linear  accuracy of 0.015 millimeters while the width of a
line on a chart is probably  around 0.5 mm in most cases --that's over 30
times larger. I wouldn't count on  angles laid out with a three-arm protractor to
be any better than 0.1 degree  accuracy. Then again, maybe I've missed
something.

As an aside,  here's a little mantra: Angles *ARE* Ratios.
An angle of one arcsecond  is a ratio of 1:206,265. An angle of one arcminute
is a ratio of 1:3438. And an  angle of one degree is a ratio of 1:57.3.
Memorize any one of these and you  never need trig for small angle calculations.

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

```
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