NavList:

   

Reply

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

   

Reply