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    Re: GPS shortcomings.
    From: Robert Eno
    Date: 2005 Jun 11, 22:14 -0400

    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
    
    
    

       
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