NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
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From: Fred Hebard
Date: 2006 Feb 28, 13:30 -0500
From: Fred Hebard
Date: 2006 Feb 28, 13:30 -0500
Greg, Welcome! Mirrors have to be very precisely leveled. There has been much discussion of this on the list, which is in the archives. Another alternative is to use a liquid and let gravity level it. Unless you are up against the shore near tall mountains, such as along the South American coast, gravity gives a very fine level surface to liquids; the same shortcoming would also apply to bubble levels used to level a mirror. I use a Davis-brand artificial horizon, and fill it with dark Caro-brand corn syrup overlain with a thin layer of vegetable oil. I'm not sure that Polaris will be easy to see in the horizon, but bright stars such as Sirius, Vega, etc are easy to see, plus the planets, sun and moon. Typically, I can get positions within 0.2-0.3' of arc, and sometimes under 0.1' of arc. That is pretty close to the limits of resolution of the sextant (0.05' of arc with an artificial horizon. For objects moving fairly rapidly across the sky, timing the sight accurately is an important component of overall accuracy. This would be less true for Polaris, should you be able to see it in the horizon. Mounting the sextant on a tripod would also facilitate precise observations. Yours Truly, Fred Hebard On Feb 28, 2006, at 12:44 PM, cfi@licfi.com wrote: > Good Day to the List! > My name is Greg & I have an interest in historical navigation. In > addition > to enjoying > sailing I belong to a local Rev. War group, where I have the > persona of a > surveyor. > Since surveyors also had to know how to determine lat/Lon I have > started to > teach myself the Basics. At this point I have hit a small snag. It > should > be pretty straight forward to shoot Polaris But living inland I > need to use > a artificial horizon a "well leveled mirror" as the books put it, > and I > can't seem to come any closer than 3-4 NM. The sextant I am using is a > MAC(" Measure All Co. " - Japan ) that I bought off of a retiring > blue > water boater on eBay; it seems to be in quite good condition, and I > think > $250 was a fair price. All I can think is that there is something > wrong > with the way I am setting it up. May I ask for some guidance please: > > > Why does the star look "smeared" through the mirror & 'OK' > during > a normal horizon sight? > > How optically flat does the mirror really need to be? > > How close (or far away) from the mirror should you need to be? > > Is a good quality carpenter?s level accurate enough? > > > Many thanks to the list in advance! > -Greg > > > > -------------------------------------------------------------------- > mail2web - Check your email from the web at > http://mail2web.com/ .