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    Re: Weems and Plath Celestial Slide Rule
    From: David Pike
    Date: 2020 Apr 12, 02:18 -0700

    Ed J you wrote:  I understand that the purpose must be to relate GHA to LHA for whatever body you are looking at, but it has 4 degrees of freedom so not sure what they meant it to simplify.

    Thankyou for the much better photograph.  The device is beginning to make more sense.  It’s essentially the same as I described except the function of the blue and white discs have been transposed.  I’ve amended my instructions to allow for this and will post them separately. 

    The symbol above zero on the white SHA disc is the Aries symbol indicating the first point of Aries or the Vernal Equinox, the point on the celestial horizon where SHA is measured from in a westerly direction.

    The t, or possibly lower-case tau, on the amber disc and cursor is, in my opinion, somewhat ambitious.  There are at least three methods of describing the position of a body along the celestial horizon with respect to the observer’s meridian: Local Hour Angle (LHA) measured in degrees up to 360 west of the observer’s meridian; Median Angle measured in degrees up to 180 east or west of the observer’s meridian;  and in hours, minutes, and seconds up to 24 hours west or 12 hours east or west of the observer’s meridian.  However, there’re no 24 hour divisions on the amber disc, hence my suggestion that the use of the symbol ‘t’ is ambitious.  I might be that ‘t’ is the acknowledged symbol for longitude, hour angle, or median angle; I’m no an astronomer.

    As to four degrees of freedom.  I’m assuming you mean three moveable discs and the cursor.  The cursor doesn’t apply an input, it’s only there to help line the other values up easily, so perhaps it’s only three DoF.

    As to use.  I stick to my original suggestion that it’s meant as a confidence booster for those who get little chance to practice, or as a checker for use when you’re working under adverse conditions and feel you might make a mistake with your maths.

    The LHA/Median Angle converter on the amber disc, is probably least useful, because meridian angle rarely figures in modern navigation text books. DaveP

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