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Lu Abel writes:
[...]
 > I personally wonder if this IE drift would be an issue for a voyager, who
 > is hopefully surrounded by both decent breezes and the thermal mass of the
 > ocean.

I found Larson's assumptions somewhat suspect or at least procedurally
weak.  For example, I would have more confidence in his conclusions if
he had each student using both sextant types.  There doesn't appear to
be any control for the optical and design (e.g., mirror size and type)
differences.

But, procedure aside, he's otherwise reporting the obvious.  Take a
sextant from one environment to another and it's subject to thermal
stress.  Some materials and designs handle this stress better than
others.  Keep a sextant in a constant environment and there's less
shift due to thermal stress.

Thermal damping from the ocean won't completely eliminate or offset
local thermal inputs from insolation.  It will help to smooth out the
overall air temperature in the course of the day, of course, but
sights taken on land in 85 deg. air will see the same heating (all
other factors remaining constant) as sights taken at sea in 85
deg. air.  Air movement will have some influence, of course, but, for
example, a breeze in 85 deg. air is still not as effective as removing
heat in 70 or 60 deg. air.

Bottom line: sextant materials and design are still an overwhelmingly
important factor.

Rick
S/V One With The Wind, Baba 35

   

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