Re: [Leica] plastic verses metal (sextants)

[John Collier <jbcollier@home.com>]

A few comments. This is entirely anecdotal and subjective, the writer does
not even say if they have actually compared metal and plastic sextants under
similar conditions. Second, it is possible to tailor the composition of
synthetics to suit many thermal requirements, just ask NASA. Even the
assumption that "metal" is heavier is not correct. It depends on the metal
in question. I understand what the writer is trying to say and probably even
agree with them but the quality of their arguments is very poor.

John Collier

> From: "Alan Hull" <hull@telia.com>
> Reply-To: leica-users@mejac.palo-alto.ca.us
> Date: Mon, 11 Sep 2000 08:33:20 +0200
> To: <leica-users@mejac.palo-alto.ca.us>
> Subject: [Leica] plastic  verses metal (sextants)
> 
> Luggers
> The following is from the Cruising World magazine website.  It is
> about plastic verses metal sextants for astro-navigation.  Sound
> familier?  Substitute photo for sights.
> 
> http://sailingworld.com/2000/06/Sextant.htm
> I knew that subjectivity would play a large part in our experiment, as
> it does in real life. I take better sights with an instrument I love.
> The subjective factors that affected our sights included our
> experience, the weight and the feel of the units, the quality, size and
> colors of the filters, the orientation of the index lever, the feel of
> the micrometer drum as well as fatigue, impatience, nervousness,
> sleepiness, or whatever emotional or physical state we were in that
> day.
> 
> I’d have to stand by my disparagement of the plastic sextants. My
> experience and distant memories of physics classes tell me that
> different thicknesses of plastic will heat and expand at different
> rates. That plastics are affected more quickly by heat than metal I
> would take to be a given
> 
> As for the inertial properties of various weights of sextants, there
> was this guy named Newton who mentioned that an object at rest tends to
> remain so. I have found that a lighter object will be easier to start
> moving than the heavier and therefore more difficult to hold steady so
> I’ll stand by my preference for a heavier sextant in a seaway when the
> boat is leaping around.
>