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From: Frank Reed (no email)
Date: Wed Oct 12 2005 - 19:38:19 EDT
About two weeks ago, George H wrote:
"I've seen that sentiment expressed before, by other seasoned professional
mariners, and I would like to understand the basis for it. Why should a
heavy sextant be more stable, I ask (as someone who only ever owned a
plastic sextant, but has used others)? I respect the views of those who
have much more experience than I do, but remain as yet unconvinced.
After all, the windage on a sextant is just the same, when made of a light
material or a heavy one, if they are the same shape and size. If a sextant
is physically smaller, as some yachtsman's models are, then wind forces
will be less. True, a heavy sextant will have more inertia, so it resists
initial movement, but when moving with the ship's roll, then it acquires
extra momentum, which makes it harder to bring it to a stop.
Sextant makers all seem to go to a lot of trouble to skeletonize the frames
of their instruments. If there was as advantage in having a heavier
instrument, why would they bother to do so?
The clinching argument, to my mind, against Willem's view, is that if
weight was a real advantage, mariners would "improve" their lightweight
instruments by simply adding lead ballast, to make the thing more "stable".
I have never heard of this being done, to any sextant, though it would be
easy to do in practice. Why not, then, if the extra weight would make it
somehow better? Convince me that it would. "
I thought this was an interesting post and intended to comment on it but I
have only just now remembered it. Maybe some others will have some thoughts. I
agree with your theoretical point, George, and so I've been trying to think
what it is about an instrument with a little "heft" to it that makes it
easier to use. My best guess for now is that this is a feedback and control issue.
The muscles of the hand and forearm may be able to control a somewhat
heavier sextant because of the feedback its inertia gives (more specifically, its
moment of inertia). I think this also helps damp out hand tremors (which I
experience). Of course, at a certain point, the instrument would be too heavy
for most people to hold comfortably so there should be some optimal "middle"
weight. Just a thought...
Why don't people weigh down lightweight plastic sextants with lead ballast?
Maybe for the very simple reason that no one thought of it before. It might
very well be a good idea. Three little lead weights at the extreme corners
might give good stability without adding much weight.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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