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    Re: Sextant Error and Whole Horizon Mirrors
    From: Bruce Stark
    Date: 2002 Jul 11, 11:41 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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