NavList:

Red wrote:
> Good points, Lu.
>
> "Aluminum has replaced brass as the material of choice for sextants,"
>  How does it compare for thermal expansion, or rather stability?

That's beyond my knowledge of metallurgy.  But one thing to remember is
that a sextant is really a high-precision protractor for measuring
angles.  If everything expands uniformly, all I have is a slightly
larger protractor which would still measure angles correctly (unlike a
"slightly larger" tape measure).


> Incidentally, the top of the Washingotn Monument is capped with a block of
> aluminum, which was at that time the most expensive metal on earth. I wonder if
> "engineering resins" or ceramics of some type would have better thermal
> stability than metals today? (And still be suitable in other ways.)

I believe that was the whole premise behind the Davis and Ebbco plastic
sextants.   Plastics are mostly used as a cost-cutting measure (think of
the plastic bumpers on our cars these days) so "plastic" has become
synonymous with "cheap but not necessarily good."   But there are
"engineered plastics" that are strong and stable.  In fact there's at
least one boat accessory company that's manufacturing boat through-hull
fittings of plastic instead of metal as a solution to the galvanic
corrosion problems inherent with metal through-hulls.

It would be really interesting to see how far one could take a plastic
sextant in terms of precision, longevity, and low cost.  Unfortunately,
there's not the demand to make it happen.

Lu

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