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Jared you wrote:
"But in theory a good  navigator would allow a sextant to acclimate to
heat/cold and then recheck  the index error before using it."

Yes, I agree. You absolutely should do  this, and it's even a good idea with
a metal sextant if the temperature is  extreme. When it's near zero degrees
(Fahrenheit --yes, cold) even a metal  sextant will change its IC as it cools
off, in my experience. But even when a  plastic sextant has been allowed to
reach ambient temperature, you can still  expect relatively worse results than you
will get from a metal sextant. In  practice for typical expectations for
celestial navigation, it's not really a  big deal. You can even shoot demanding
sights like lunars with a plastic sextant  if you don't mind errors as big as a
couple of minutes of arc (corresponding to  an error of one degree in
longitude).

By the way, plastic sextants also  seem to have significantly larger shade
error than metal sextants. With some  patience, you can measure and record this
error for each shade and apply it to  your sights. It'll help, but it's still
a plastic sextant.

If you want a  historical parallel, at least into the first third of the 19th
century, many  navigators at sea carried octants made of wood, often ebony,
and a sextant made  of metal. The expensive, delicate, and accurate sextant was
specifically  reserved for shooting lunar distances while the cheaper, less
accurate octants  were used for shooting ordinary altitudes. If you ever find
yourself in a  position to "demo" lunars or other historical navigation
techniques, you might  use a plastic sextant as a stand-in for the old octant.

-FER
42.0N  87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars

   

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