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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


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