NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: GPS shortcomings.
From: Robert Eno
Date: 2005 Jun 11, 22:14 -0400
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