 # NavList:

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

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From: Paul Hirose
Date: 2016 Nov 12, 21:55 -0800

```On 2016-11-10 13:33, Bob Goethe wrote:
> Attached File: http://fer3.com/arc/imgx/_Mark-2-Slide-Rule-User-Manual.pdf

On the Bygrave form on page 7 I find the format of the formulas
unnatural, though their meaning is perfectly clear. Isn't it usual for a
variable which receives a value to be on the left? E.g., x=2 not 2=x.
That's the way it works in all programming languages I've seen. But on
the Bygrave form the assignment goes from left to right.

On the same page, there's a problem with the rule that determines
whether azimuth angle is measured from the north or south. Occasionally
it's wrong, or maybe I don't understand it. Below are three examples. In
each case Z itself is correct, but since declination is south of the
latitude (but both are north of the equator), I put a "south" prefix on
Z. That results in a wrong azimuth.

LHA = 84
H = 84
L = 30
d = 29
W = arctan(tan d / cos H) = 79.3
X = 90 - L + W = 139.2
Y = 180 - X = 40.7
Z = arctan(cos W / cos Y * tan H) = S 66.7 W
Hc = arctan(cos Z * tan Y) = 18.8
Correct Z = N 66.7 W, Hc = 18.8.

LHA = 307
H = 53
L = 45
d = 41
W = arctan(tan d / cos H) = 55.3
X = 90 - L + W = 100.3
Y = 180 - X = 79.7
Z = arctan(cos W / cos Y * tan H) = S 76.7 E
Hc = arctan(cos Z * tan Y) = 51.7
Correct Z = N 76.7 E, Hc = 51.7.

LHA = 286
H = 74
L = 41
d = 30
W = arctan(tan d / cos H) = 64.5
X = 90 - L + W = 113.5
Y = 180 - X = 66.5
Z = arctan(cos W / cos Y * tan H) = S 75.1 E
Zn = 104.9
Hc = arctan(cos Z * tan Y) = 30.5
Correct Z = N 75.1 E, Hc = 30.5.
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