# NavList:

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

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Re: Can You Bias Ho?.....Misunderstood 2ed question
From: Greg B
Date: 2014 Apr 04, 10:39 -0400
All,
I believe you have misunderstood my 2ed question, which had nothing to do
with the first Ho bias question ( but THANK YOU - lots of good info & insight )
In attempting to recreate the techniques of Lewis & Clark,
I am trying to understand how they measured latitude.
It is my understanding that when it was impractical or impossible to do a noon sun
sight they used some of the brighter navigational stars at meridian passage.
I know they did not use a Hg AH, but a mirror leveled with a spirit level.
What I don't know is how critical is meridian passage, angle wise;
i.e.  ( 180 degrees +/- ? degrees is acceptable? what order magnitude is it?)
How did they know when they had meridian passage? On the surface, It seems
it would be the same as trying to do noon sun for latitude - but I get the feeling
there was more to it.
Thanks,
~Greg

On 04/04/2014 04:31 AM, Luc Van den Borre wrote:

```On 4/04/2014 3:11, Brad Morris wrote:
> Anon, like me, can see no use for the up arrangement of the pentaprism.
> I still don't understand why Nicolas would do that.  He's a bright
> fellow, I'm sure he had a reason.

It's a surveying sextant, meant to be used horizontally.

If a sextant's normal range is -5° to 125°, the range with the
pentaprism 'pointing down' will be 85° to 215°, so if the prism were
fixed you wouldn't be able to measure angles smaller than 85° without
unscrewing the prism housing from the frame.

With the pentaprism in the other position the range will be -95° to
35°, returning the ability to measure angles smaller than 85°.

Both the C&P and Freiberger pentaprism attachments can be rotated 90°,
of course:

http://www.cassens-plath.de/katalog/index.html?startpage=90
http://www.fpm.de/index.php?c=1&s=pentaprisma

Luc
```

View and reply to this message: http://fer3.com/arc/m2.aspx?i=127443

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