NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
05-10+11 Lunars + scope adjustment
From: Fred Hebard
Date: 2003 May 9, 15:26 -0400
From: Fred Hebard
Date: 2003 May 9, 15:26 -0400
Doug, Regarding the alignment telescope. This procedure is described also in Bowditch, without the wires. Again, Bowditch says to chose two stars at least 90 degrees apart and to note the angle, but no further mention is made regarding use of that angle. I presume then that one notes the angle to be sure it is at least 90 degrees. Perhaps others have more knowledge of this? One would bring the stars into confluence at one wire, and then check that they are still in confluence at the other wire. Without the wires, Bowditch says to bring two stars into confluence at one side of the scope and then move them to the other side to check that they retain confluence. Whether they separate or overlap would then tell one in which direction to adjust a scope that was out of alignment. I don't think modern sextants have provision for making this adjustment in the field. The wires may originally have had another use. Similar wires used to be located in land-based astronomical telescope eyepieces. One would note the time when celestial objects crossed each wire, to better compute ephemerides and such. This may also have been the original purpose of the sextant wires, for land-based observations using an artificial horizon. Perhaps others can enlighten us as to the purpose of the wires. Simex or Takizawa, sextants have alignment scopes with two wires. Fred