NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant precision
From: Robert Gainer
Date: 2004 Sep 30, 14:27 +0000
From: Robert Gainer
Date: 2004 Sep 30, 14:27 +0000
This is a sharp group of people. I see Alexandre has had something to say about the ice before I finished composing my note in word. If I just sit back and read everthing coming by I can become very much smarter, maybe. The only time that I have heard of using an artificial horizon other then the type you can buy from Plath is on a Ship in an ice field. There would be no movement of the ship under those circumstances. I have been on ships and yachts and even in a dead calm the yachts roll too much to even consider a pan of anything. The ships will also roll and I can�t imagine any one having success with an oil filled pan. The Plath artificial horizon is built into a scope and would be useful under the right conditions. One of my sextants was made by Brandis & Sons and has an arc with a radius of 5-1/2 inches to the graduations on the limb. The limb has a silver insert graduated in 20-minute increments from �5 degrees to 185 degrees. The vernier is graduated in 30 second divisions from �30 seconds to 20 minuets 30 seconds. The largest reading you can make is 162 degrees before you hit the index arm stop. I can read it to a precision of 15 seconds of arc by estimating the vernier. I have not used this instrument in 25 years and did not know how to measure the accuracy at the time. I pulled it off the self yesterday and if time permits I will test it this Sunday. My Cassens & Plath has a 6-inch arc that is graduated from �5 degrees to 125 degrees by 1-degree increments. The micrometer drum is graduated in 1-degree increments with a vernier that reads to 1/10 minute. I think I can read it to a precision of 6 seconds of arc. The instrument came with a certificate stating that it is accurate to better then + - 9 seconds of arc. Now that this question of precision and accuracy has come up I am wondering what the significance of the statement on the Plath�s eccentricity certificate is when it says the sextant has no errors in practical use. Is it conceivable that the accuracy is such that you can measure an angle in a meaningful way to the nearest 20 seconds of arc, the + - 9 seconds on the certificate. In use on the boat I am happy if my position is within 2 miles or less compared to the chart and to be less then 1 mile off I think is doing very well indeed. All the best, Robert Gainer _________________________________________________________________ Get ready for school! Find articles, homework help and more in the Back to School Guide! http://special.msn.com/network/04backtoschool.armx