NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Master & Commander
From: George Huxtable
Date: 2003 Dec 8, 08:48 +0000
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@huxtable.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. ================================================================