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From: Frank Reed (no email)
Date: Sat Jun 11 2005 - 17:41:05 EDT
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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