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Remember, LHA is in the range of 0� to 360� and is always measured to 
the west from the assumed longitude (ALon). It is the angle measured 
westward from the meridian of the ALon to the meridian containing the 
body's grographic position (GP). There is no such thing as easterly LHA.

In the past, various computations methods and tables (e.g H.O. 214) used 
"hour angle," (abbreviated "H.A." or "H" or "t"), which is the angle 
measured between the meridian of the ALon and the meridian containing 
the body's GP measured the shortest way, either west or east. Using this 
notation, hour angle ends up in the range of 0� to 180� only and is 
denoted "east" or "west." Because of the the way the trig formulas work, 
using either method computes the same Hc and the same "azimuth angle" 
("Az" or "Z"). The only thing affected by choice of notation is the 
method used for the final determination of Zn,(azimuth used for plotting 
the LOP.)

The original Bygrave used hour angle, not LHA, and the scales were 
marked from 0� to 90� and then back the other way, 90� to 180�. My 
implementation of the Bygrave eliminated the second set of markings on 
the scale, 90�-180�, to eliminate clutter so I added an extra step to 
bring hour angle into the range of 0� to 90� only and provided the 
necessary rules for the final computation of Zn. This is what is 
happening on the top of the form I provided. If LHA is less than 90�, H 
= LHA; if LHA is greater than 90� but less than 180�, H = 180� - LHA; if 
LHA is greater than 180� but less than 270� then H = LHA - 180�; and if 
LHA is greater than 270� but less than 360�, H = 360 �- LHA. 
Conceptually, this is the smallest angle measured from either the upper 
branch or from the lower branch of the observer's meridian to the 
meridian containing the body's GP.

See the revised form at:

http://www.fer3.com/arc/img/108719.revised%20form%206-18-09.pdf

The formulas for calculating LHA are:

If your AP is in west longitude: LHA = GHA - ALon. (If necessary, add 
360 �to GHA prior to subtracting ALon.)

If your AP is in east longitude: LHA = GHA + ALan. (if LHA then exceeds 
360�, subtract 360� from the result.)

Using the first formula for your first two examples, GHA (55� + 360�) - 
77� = LHA = 338�.

GHA 95� - 77� = LHA = 18�

The third example you bring up makes no sense since GHA is never 
measured to the east, it is always measured west from Greenwich.

gl


Andrew Corl wrote:
> All,
>  
> I need some help.  I am attempting to work the problem in Ocean 
> Navigator using the Lapook-Bygrave Slide Rule.  I am uncertain how to 
> compute the Local Hour Angle (LHA).
>  
> In the following cases I am assuming my longitude to be 77 degrees 
> west of Greenwich.
>  
> 1. The GHA of the Sun is 55 degrees west of Greenwich
> 2. The GHA of the Sun is 95 degrees west of Greenwich
>  
>  
> In the following cases I am assuming my longitude to be 120 degrees 
> west of Greenwich
>  
> 1. The GHA of the sun is 170 degrees east of Greenwich.
>  
> Also what is the formula if my position is east of Greenwich and the 
> sun has a GHA of more than 180 degrees?
>  
> Thanks
> Andrew
>
> >


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