# NavList:

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

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Re: Snellius Construction questions
From: Herbert Prinz
Date: 2009 Jan 05, 22:18 -0500

```The construction is based on the fact that

1. Periphery angles over the same arc are equal

and

2. The central angle is twice as large as the periphery angle.

Therefore, for any given chord, the locus of all points from which that
chord is seen under a given angle is a circle. The center of that circle
is on the bisector of the chord, exactly in the place from which that
chord is seen at twice the given angle.

Of course, said circle is a LOP. (The claim on the referred website that
a position can be derived without LOPs is misleading.) In order to get a
position, one must combine this circle with any other suitable LOP,
which may or may not be obtained in the same way.

The horizontal angle between the two chosen landmarks can be measured
directly with a sextant or as the difference of two compass bearings.
The former has the advantage of increased accuracy (minutes of arcs
instead of degrees). I believe there is a plotting tool specially
designed to assist with the method. I have never seen one in natura, but
it's some kind of a rule-plus-protractor with three arms that can be set
to the given angles.

Herbert Prinz

NavList@aol.com wrote:

>
>I have found an intriguing example of triangulation from a 16th century
mathematician, Willebrord
Snellius(http://www.sailingissues.com/navcourse4.html). I have searched other
web sources on the web and so far, have had no success in finding a straight
forward explanation of how this method of triangulation works. Have any of
you actually used the "Snellius Construction" technique, and could you help
me understand it as a practical navigation application?
>
>Thank you,
>
>Todd Frye
>
>-----------------------------------------------
>[Sent from archive by: fryefamily-AT-sisna.com]
>
>
>>
>
>
>

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