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    Re: The backstaff. was: Re: The Shovell disaster
    From: Michael Daly
    Date: 2007 Nov 03, 14:02 -0400

    George Huxtable wrote:
    > And I wonder about the logic behind making one part of the arc (the 60
    > degree section) to have a much smaller radius than the other, seeing that
    > the overall reading was the addition of the two arcs, and both therefore
    > needed to be set, or read, to similar accuracy.
    The difference is reading versus setting.
    If you are reading a scale, the radius must be long if you wish to have
    fine graduations on the scale.  This is important, since the sight vane
    can end up at any position and having lots of fine graduations is only
    easy if the scale length (and radius) is large.
    If you are setting the position, as you would with the shadow vane, you
    don't need lots of graduations.  Graduations to 5 degrees are adequate.
      You can set the vane very accurately to any of these graduations since
    your vernier acuity is quite high.  Thus you don't need a large radius
    to create an accurate setting.
    Vernier acuity is the ability to determine whether two line segments are
    co-linear.  In this case, the two line segments are the graduation on
    the scale and the indicator on the shadow vane.
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