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Bill Murdoch wrote, about calibrating an octant-

>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

My comment-

Fine, if you have an accurate steel tape that's 10 metres long (and a bit),
and a garage that's much larger than most of us possess, something over 7.5
metres square, and some way of providing a curved track or groove of the
correct radius to wrap the tape around and locate it. The radius should be
struck using the steel tape, to protect against thermal expansion effects,
and thereafter measurements should be made at that same temperature.

These are only practical difficulties, not difficulties in principle, but
some of them may be hard to overcome. Scaling things down, and accepting a
lower accuracy, would make the job somewhat easier.

George Huxtable.

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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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