# NavList:

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

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Re: Journal of Navigation - latitude with a Bris sextant
From: Andrés Ruiz
Date: 2022 Sep 14, 15:00 +0200

Hello Brian,

Thanks for sharing.

After a very quick reading, some thoughts.

The paper deals with two different things: Bris sextant and Sight Reduction method.

## Bris sextant

I've built dozens of Bris sextants, it is a lot of fun to make, calibrate and use afterwards. I have used welding glass 12, I think that shade-5 could be harmful to our eyes.

Declination

In your example you assume that Dec = cte and that the observer is stationary.

dDec/dt

14/09/2022      10:30:36                3º 19.9'

14/09/2022      13:16:59                3º 17.2'

20/10/2022      13:16:59                -10º 26.9'

20/10/2022      10:30:36                -10º 24.5'

20/10/2022

10:30:36 UT1

Sun derivatives

dDec/dt = -0.014922 º/h = -0.9 '/h = -0.00 '/s

dGHA/dt = 15.001708 º/h = 900.1 '/h = 0.25 '/s

And in your simulation Dec(-25, +25), looks weird.

## Latitude

Seems that you use a variation of “Latitude by doble altitudes an elapsed time”, or Douwes, (Chauvenet 211, Pg 315), where Latitude = f(t1, Dec1, Ho1, t2, Dec2, Ho2) or Latitude = f(t1, Dec1, Ho1, t2, Dec2, Ho2, DR_latitude)

## Longitude

If dDec/dt and dB/dt, dL/dt, (MOO motion of the observer), are negligible the altitude/time curve is symmetrical with respect to the meridian passage. Otherwise the error could be important.

## Fix

Using a Bris I always use the MSH intercept method.

For example one of the simplest ways to obtain the position is: “Celestial Fix - 2 LoP analytic solution“ available at https://sites.google.com/site/navigationalalgorithms/Home/papersnavigation, that takes into account for Running fix.

Fair winds.

Bris sextant [es] [en]

El mié, 14 sept 2022 a las 2:02, Brian Villmoare (<NoReply_Villmoare@fer3.com>) escribió:

I thought some folks might find this interesting. I published a method for calculating latitude using a Bris sextant in the Journal of Navigation. I would, of course, appreciate comments, questions, or criticism. If one of you were one of the reviewers, well, I suppose this isn't new to you!

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--
Andrés Ruiz
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