# NavList:

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

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Re: slide rule sight reduction accuracy
From: Gary LaPook
Date: 2009 Jun 18, 03:07 -0700

Here is  the promised revised form.

gl

Gary LaPook wrote:
```I have attached a revised form that carries the azimuth rules used with
my version of the Bygrave. These are slightly different than the
original rules with the original Bygrave due to my simplification of the
scales. I omitted the second set of numbering on both scales, those
running from 90 to 180, to keep the scales uncluttered. The results are
the same as a value exceeding 90  will be found at exactly the same
point on the scales using my method or using Bygrave's. For example, LHA
150, Hour Angle (H) 150 west on the original Bygrave will be found at 30
on my version, the same point that is also marked with "150" on the
original Bygrave. The same thing happens for values on the cosine scale.

The original Bygrave produced azimuths in the range of 0 to 180 and are
always laid off from the opposite pole while with mine the azimuth is
determined in the range of 0 to 90 and are laid off from either pole as
determined by the rules on the form.

Here is a link to my original post:

http://www.fer3.com/arc/m2.aspx?i=107414&y=200902

(Note I made an error in the explanation of the sample problem, the
declination is 20 north not 14 north. I also used a form for the
illustration using Byrave's "y" instead of my "W".)

Due to my simplified scales I introduced on additional intermediate
value "X" and, for clarity's sake, I replaced Bygrave's lower case "y"
with "W". Then using "W" with "X" I derive a "Y" in restricted to the
range of 0 to 90. Using my method computes the same values as when using
the original Bygrave.

Notice that "X" determines the final Zn. If "X" exceeds 90 then Zn is
laid off from the nearer pole and from the far pole when "X" is less
than 90.

Here is a link to the Bygrave manual:

http://www.fer3.com/arc/img/107419.bygrave-manual.pdf

gl

Paul Hirose wrote:
```
```Regarding the Bygrave simulation, it can be done if I know the rules.
For instance, azimuth is obtained by taking an arctangent, but that
function yields a result between -90 and +90. There must be some rule to
convert that to the range 0 to 360.

```
```

```

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