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Subject: Re: Sextant Error and Whole Horizon Mirrors
From: Bruce Stark (Stark4677@XXX.XXX)
Date: Thu Jul 11 2002 - 11:41:34 EDT
Aubrey-
That's great! I wouldn't have believed they were making inverting 'scopes for
sextants that recently. No doubt a good deal of experience is required, but
those who used them liked them. Lewis and Clark preferred the inverting
telescope.
What follows is from a 1796 Moore's New Practical Navigator. For the
convenience of the modern reader I've gotten rid of the S's that look like
F's, typing "sextant" instead of "fextant", "sun" instead of "fun", etc.
Here's what Moore had to say:
"The method of doing this is, making observations of angles as much above 90°
as possible, using the inverted telescope: in doing of which the wires in the
focus of the eye-glass must be first placed parallel to the plane of the
sextant, after which bring the sun and moon into very nice contact, on the
centre of the wire nearest the sextant, taking care that the index is not the
least moved, then direct the telescope in an instant so that the images may
appear in the centre of the outer wire which is farthest from the sextant,
and if the contact appear the same at this wire, the axis of the telescope in
parallel to the plane of the sextant; on the contrary, if the limbs of the
objects appear to separate at the wire which is farthest from the sextant, it
is plain the object-end of telescope is too far from the plane of the
sextant, and must be corrected by turning one of the two screws of the ring
into which the telescope is screwed and fixed, having previously unturned the
other screw.
"If the limbs of the images overtop each other at the aforesaid wire, it
shews that the object-end of the telescope is too near the plane of the
instrument, and consequently must be brought parallel, by means of the
screws, and this method must be pursued until the distance of objects are
found perfectly the same at each wire, which stands equi-distant from the
centre of the telescope and parallel to each other, in which case the axis of
the telescope will be exactly parallel to the plane of the sextant, and then
you may proceed to measure the angles for the observations, observing to
bring the limbs of the objects in exact contact, in the centre between the
two wires before-mentioned."
Enjoy your sextant!
Bruce
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