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Paul Hirose and Dick Winchurch, thanks.

I now think I understand how the instrument error in a homemade octant could
be measured.  It is much easier than I had thought.

I got stuck on trying to measure the angle between the sight line to the
horizon and the sight line to the body.  That would be hard to do because the
intersection of the two lines occurs behind the octant and moves as the index
mirror moves.  To make the intersection of the two sight lines occur in the
middle of a measuring circle, the octant would have to be relocated each time
the index arm was moved for a measurement.  That would not be an easy task.

The way you describe is so much better.  Fix the octant body with its index
mirror in the center of a circle with the perimeter of the circle marked in
minutes of arc.  That puts the sight line from the index mirror to the body
on a circle radius and puts the horizon sight line on a line offset from, but
parallel to, a circle radius.  Setting the octant to zero determines the
location of both the horizon target and the body target on the perimeter of
the circle for an octant reading of zero.  At zero the targets are separated
by the distance between the telescope and the index mirror.  During the
calibration neither the octant frame nor the horizon target is moved.  The
body target is moved along the calibrated perimeter of the circle, and its
position is measured by moving the index arm of the octant and reading the
scale on the arc of the octant.  A comparison of the change in the position
of the body target on the circle perimeter with the arc reading would give
the instrument error.

I think I can do it in my garage like this:  (1) Fix a block of wood on the
floor with a 1/4" hole drilled in the wood.  (2) Strike an arc just over 100
degrees in length with a radius of 6,875.5 mm centered on the hole.  (3)
Stand a steel tape measure on edge along the arc so that each 1 minute of arc
is 2 mm on the tape.  Replace the pivot bolt in the octant with a longer bolt
and place the octant over the block of wood so that the bolt slips into the
hole.  That places the index mirror over the center of the circle.  (4) Read
the steel tape scale in the horizon glass for reference.  That reading
becomes the horizon.  (5) With the index arm set to 0 degrees read the steel
tape scale at the spot that is superimposed over the horizon reading.  That
begins the calibration.  (6) Move the index arm and note the scale reading
viewed in the index mirror that is now superimposed over the horizon reading.
 If the scale can be read to a half millimeter, the sextant error can be
measured to 0.25 minutes of arc.  That is four times better than my usual
shooting accuracy and I think good enough for checking a homemade instrument.

Think that would that work ?

Bill Murdoch

   

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