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    Re: Master & Commander
    From: George Huxtable
    Date: 2003 Dec 8, 08:48 +0000

     Cliff Sojourner said-
    
    >one more nit in the scene with the officers shooting the noon sights
    >with their octants:  they are all on starboard, facing outboard ... sun
    >to their backs!!!
    >
    >over-the-shoulder shots with octants, that's a real trick!!
    
    =========================
    
    From George-
    
    Not so crazy a notion, but only in special contexts. Octants measured only
    up to 90 deg., so were unsuitable for some wide-angle lunars and for
    reflected altitudes measured with an artificial horizon on land when the
    altitude exceeded 45deg.
    
    To extend the range, many octants were fitted with an extra sighting
    peep-hole on the opposite side of the A frame to the normal peep, and
    rather lower down, with its own horizon mirror. To use it in "back
    observation" mode, you turned the octant round to view it from the other
    side, and the Sun light entered the horizon mirror behind the observer's
    back. The second peep was low down to allow room for the observer's head,
    without it getting into the light path. The scale readings of 0deg to 90
    deg then corresponded to angles of 180deg to 90 deg. The big difficulty was
    measuring the index error, in this mode of use.
    
    It could have an occasional application, in measuring Sun altitudes, if
    land nearby prevented a view of the horizon under the Sun. Then an observer
    could measure the altitude of the Sun from the horizon in the opposite
    direction, an angle greater than 90deg, using an octant with a back
    observation, and putting the Sun behind his back. "Over the shoulder", as
    Cliff describes it. Later, an unmodified sextant could be used in a similar
    way, as long as the Sun's altitude exceeded 60 deg. As sextants got
    smaller, however, the observer's head would block the light path.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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