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    Re: recommendation for slide rule ?
    From: Gary LaPook
    Date: 2009 May 25, 13:57 -0700

    I have now worked some sample problems using Bygrave's formulas on a
    ten inch rule and it does seem to be an improvement over the sine-
    cosine method since the tan scale has better resolution. It is also
    obvious that a slide rule with two tan scales  would be easier to use
    for this computation.
    
    gl
    
    On May 24, 2:48 pm, glap...{at}PACBELL.NET wrote:
    > After reading Dave Walden's post on this thread on May 20 about using
    > the Bygrave formulas on the ten inch slide rule, it occured to me that
    > that may be the better method since it uses tans with the advantages
    > that you point out. I haven't tried this out yet but it makes sense to
    > me.  BTW your matrix math left me in the dust, I don't remember that
    > part of my math education.
    >
    > gl
    > On May 23, 9:00 pm, Paul Hirose  wrote:
    >
    > > I decided to work Gary LaPook's sight reduction problem on a slide rule
    > > by means of rectangular coordinates.
    >
    > > The inputs are lat. = +34°, LHA = 14°, dec. = +20°.
    >
    > > First, convert spherical coordinates LHA and dec. into rectangular
    > > coordinates in a system whose +z axis coincides with the north pole and
    > > +y axis intersects Earth's axis. This requires 1) negating LHA then
    > > subtracting 90° to obtain an angle "theta", -104°, which conforms to the
    > > usual spherical convention, then 2) converting theta and dec. into xyz
    > > coordinates via these formulas:
    >
    > > x = cos dec. cos theta = -.2274
    > > y = cos dec. sin theta = -.911
    > > z = sin dec.           =  .342
    >
    > > Form a 3x3 rotation matrix to convert the above vector to the observer's
    > > horizontal orientation. This requires an x rotation by (90° - lat). The
    > > matrix is:
    >
    > > 0      0          1
    > > 0   sin lat.   cos lat.
    > > 0  -cos lat.   sin lat.
    >
    > > Multiply the rotation matrix and the vector. That yields a new vector
    > > which represents the body in a system whose +x axis is directed east, +y
    > > north, and +z to the zenith:
    >
    > > x = -.2274
    > > y = -.226
    > > z =  .946
    >
    > > It's easy to blunder when doing this by hand; a good check is to compute
    > > the sum of the squares of x, y, and z. It should be near 1.000:
    >
    > > .0518 = x squared
    > > .0511 = y squared
    > > .895  = z squared
    > > -----
    > > .9979
    >
    > > Also take the square root of the sum of x squared and y squared. This
    > > value, which I'll label d, is the distance of the body from the +z
    > > (zenith) axis:
    >
    > > .321 = d = √(.0518 + .0511)
    >
    > > Altitude is the arc tangent of z/d. Set the C scale left index to d
    > > (.321), the hairline to z (.946) on scale D, and read 71.28° (71°17') at
    > > the hairline on scale T. That is, if your slide rule has a double T scale.
    >
    > > With a single T you must compute the arc cotangent of d/z. Set C left
    > > index to .946, hairline to .321 on D, read 71.28° on the T scale *red*
    > > numbers. Had the altitude been less than 45°, the computation would have
    > > been the same as with a double T scale, and you'd read the angle on the
    > > *black* numbers. This is an example of how a double T scale simplifies
    > > the slide rule. The physical moves are no easier, and the result is no
    > > more accurate, but you don't have to think as hard. The rule itself
    > > isn't any more "powerful". (Having said that, I'm surprised so many high
    > > end rules have a single T. This includes the Keuffel & Esser "Deci-Lon"
    > > and Post "Versalog", both flagships for their companies to the end of
    > > the slide rule era.)
    >
    > > To obtain azimuth, compute the arc tangent of y/x. Disregard the signs
    > > at first. Set the C right index to .2274, the hairline to .226 on D,
    > > read 44.9° at the hairline on T. Since x and y are both negative, the
    > > actual angle is in the third quadrant of the Cartesian system, and its
    > > value is 44.9° - 180°, or -135.1°.
    >
    > > But remember the orientation of the coordinate axes. Zero degrees is
    > > east, and increases north. To change this to the normal convention,
    > > negate the angle and add 90°, resulting in 225.1° for azimuth.
    >
    > > Compared to HO 229, the slide rule solution is off .4' in altitude,
    > > perfect in azimuth. That's partly good luck. (In case you're wondering,
    > > nothing was fudged to make it come out right, and those numbers are my
    > > one and only attempt.) However, accuracy also comes from arranging the
    > > calculation so the results come out as arc tangents. Look at the T scale
    > > on a slide rule. The most compressed portion is at 45°. Even there, it's
    > > graduated every .2° on a 10 inch rule.
    >
    > > Then look at the S scale. At small angles it has practically the same
    > > resolution as T. But at 30° it's clearly more compressed, and at 70° the
    > > graduations are single degrees, tightly packed. Clearly, accuracy is
    > > going to be highly variable if you read results as arc sines.
    >
    > > With the non-inverse trig functions (e.g., sine rather than arc sine)
    > > things are different. Basically, you can obtain these to constant
    > > accuracy for all angles. Say, to .1% on a 10 inch rule. And
    > > multiplication and division also have nearly constant relative error
    > > everywhere on the scales. It's these things, plus avoidance of arc sine
    > > and arc cosine, that give the vector method its accuracy.
    >
    > > Of course the disadvantage is the greater quantity of computation.
    > > However, the pencil and paper work is simple addition of numbers at
    > > slide rule accuracy. For instance, multiplying the rotation matrix times
    > > the vector requires four sums from a total of nine numbers, including
    > > the check I suggested.
    >
    > > The choice may come down to where you're more comfortable. To do the
    > > above computation with spherical trig I'd have to look in a book!
    >
    > > --
    > > I filter out messages with attachments or HTML.
    >
    >
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