Next message: Lu Abel: "Re: Plastic vs Metal"
I agree. In fact I acclimatize my sextant no matter what the temperature but
this is especially necessary with sub-zero temperatures where the IC can
change very significantly when brought from room temperature to -30 C.
Under such conditions, it is necessary to allow your sextant to sit outside
for at least 15 minutes.
Robert
----- Original Message -----
From: "Frank Reed" <>
To: <>
Sent: Saturday, June 11, 2005 5:41 PM
Subject: Re: GPS shortcomings.
> 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
