# NavList:

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

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Re: The Bygrave Slide Rule
Date: 2009 May 28, 17:20 -0700

```Gentlemen

On my MHR-1, the outer scale increases from left to right, from 0 deg to 89
deg 40 min and then REVERSES direction and increases from right to left from
90 deg 20 min to 180 deg.

On my MHR-1, the inner scale increases from left to right, from 0 deg 20 min
to 89 deg 40 min and then REVERSES direction and increases from right to left
from 90 deg 20 min to 179 deg 40 min.

This permits large LHA or T angles.  In fact, the Cylindrical Bygrave can
compute to lower meridian crossings!  At high latitudes, a celestial object
can move with the earths rotation and still be visible throughout the 24 hour
period.  The Cylindrical Bygrave can compute that circuit.  I am not sure if
the Flat Bygrave has continued the high angle labeling, although there is no
inherent limitation as it can always be added.

Sample
Declination S 75 deg 32 min
t            145 deg 16 min
Lat         S 67 deg 33 min
Colat         22 deg 27 min
X            101 deg 58 min
Y            124 deg 25 min because same name, we added!
Az           165 deg 44 min
h             54 deg 44 min

This may have impact on the formulation of the scales discussion.  For the
outer scale: Log(abs(cos)) is symmetric about 90 degrees, as is the secant,
log(abs(1/cos)).  For the inner scale, log(abs(tan)) is symmetric about 90
degrees, as is the cotangent, log(abs(1/tan)).  If the absolute value is not
obtained, then the symmetry is lost, as many of the trig values become
negative.

Best Regards

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```
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