NavList:

Continuing from my previous post:
Firstly, a correction.   
For “Unfortunately, the body observed could be at any one of 360 longitudes depending upon the time of observation”
Read: “Unfortunately, the observer could be at any one of 360 longitudes.”

Secondly, a caution.  For any simple analogue tool there are invariably two or three equally correct ways of using it to come up with the same result.  Everybody is convinced that their method is the best, but really, it’s just which works for you.

One method of calculating local hour angle (LHA) of the Sun, (Moon or a planet).
i.   Go to the appropriate date and time in your Nautical Almanac and note the Greenwich hour angle of the Sun. 
ii. Move the blue (GHA) disc so that 350° is under the hair line of the cursor. 
iii. Move the yellow disc until your longitude (060°W say) is under the hairline.   
iv. Swing the cursor so that the hairline lies over zero on the yellow disc.
v.  Read LHA Sun from where the hairline crosses the blue scale.

E,g.  GHA Sun = 350°, longitude is 60°W, your LHA Sun is 290°.  But it’s easy.  It’s just LHA Sun = GHA Sun -longitude west or + longitude east. 

When it comes to stars, there’s not room in the Nautical Almanac to list the GHAs for all 57 navigational stars, so we list the GHA for just one point on the Celestial Sphere and convert this to the GHA of the required star.  The reference point is called the First Point of Aries.  It’s represented in diagrams by a little ram’s horn.  It’s where the equinoctial crosses the celestial equator on its way north.  The difference in celestial longitude between the First Point of Aries (Aries) and the star is called the star’s sidereal hour angle (SHA).  It’s measured from Aries to the star in a clockwise direction.  A list of the SHA for each of the 57 navigational stars can be found in the Nautical Almanac.  Therefore, GHA Star = GHA Aries + SHA Star, and LHA star = GHA star plus or minus longitude east or west.

One way of using the WR calculator to get LHA star might be as follows.
i.   Place the hairline over SHA star on the white scale.
ii.  Move zero on the blue scale under the hairline.
iii. Move the cursor so that the hairline lies over GHA Aries on the blue scale.
iv.  Place your longitude on the yellow scale under the hairline.
v.   Move the cursor until the hairline lies over the zero on the yellow scale.
vi. Read SHA Star off the white scale. 

E,g.  SHA star = 053°, GHA Aries = 077°.  So, GHA star = 053°+077° = 130°
but longitude = 040°W, so LHA star = 130°-040° = 090°

So, what about ‘t’?   ‘t’ is called the meridian angle.  It’s the angle between the meridian the star lies on and the meridian of the observer.  It’s also the angle P in the PZX triangle that we use in celestial spherical geometry calculations.  I’m not familiar with this slide rule method of getting t or P.  I just draw a diagram of a sphere and draw lines on it.  However, there are two PZX triangles for any pole, star, and observer position.  We prefer to work with the smaller one.  So, to get t or P if LHA star is less than 180, we would simply use t=LHA.  If LHA is greater than 180,  We would use t = 360-LHA.  You could calculate this using the two scales on the yellow disc.  I can’t imagine anyone doing that when you can do it so easily with a diagram, and then you know in your head what’s going on. 

The scales on this slide rule are all linear.  Therefore, all it can do is add and subtract.  If you wanted multiplication and division, you'd need logrithmic scales.  Hope this helps.   DaveP