NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Plastic sextants and prejudice. was: GPS shortcomings.
From: Chuck Taylor
Date: 2005 Jun 10, 19:32 -0700
From: Chuck Taylor
Date: 2005 Jun 10, 19:32 -0700
Just for fun I thought I would post a description of all the plastic sextants I know about. Perhaps others can add to the list. Davis Mark 3 gray plastic, solid frame "split" horizon glass, mirror + "air" vernier on the index arm, no gearing or fine adjustment vernier calibrated to 2' (that's two minutes of arc) no telescope (sight tube instead) plastic shades, frameless Davis Mark 12 black plastic, hollow frame split horizon glass, mirror + clear glass vernier on the index arm with knob for "fine tuning", no gearing on limb. reminiscent of old "clamp-screw" verniers except that the "clamp" part is squeeze- and-release vernier calibrated to 2' of arc plastic optics in the scope plastic frames around the shades Davis Mark 15 black plastic, hollow frame split horizon glass, mirror + clear glass drum vernier, calibrated to 0.2' of arc gear teeth on limb, worm gear in drum plastic optics in the scope comes with a sight tube also plastic frames around the shades Davis Mark 20 same as Mark 15, except it has a light to read the vernier and limb, powered by batteries in the handle can't remember whether it is gray or black plastic and don't have one to look at Davis Mark 25 same as the Mark 20, except for a whole-horizon mirror (called a "beam converger" by Davis) definitely gray plastic The newer Mark 25's have an LED instead of a bulb for lighting the scale Ebbco black plastic, mostly solid frame "split" horizon glass, mirror + clear glass drum vernier, calibrated to 0.2' of arc plastic optics in telescope framed shades I sort of like the Mark 12; I was getting intercepts of about 1.5' with it while at anchor a couple of summers ago. I've heard the Davis Mark 25 called a "GPS" (Gray Plastic Sextant). :-) The most striking difference between the plastic sextants and my Astra IIIb is, in my view, the quality of the optics. Another difference is that the index error tends to stay constant on the Astra. Best to all, Chuck Taylor North of Seattle __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com