NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant design. [was: Another Davis...}
From: George Istok
Date: 2003 Mar 3, 21:21 -0500
From: George Istok
Date: 2003 Mar 3, 21:21 -0500
Rodney and all, Thanks for forwarding George Huxtable's message. I purchased the Davis Instruments Mark 15 used. It appears to have been used very little. All of the adjustments on the sextant appear intact, the index is positive with no play, and the micrometer is the same. There is an error in the instrument in that the mark on the frame and the mark on the arm do not align the same through the arc. The error is visible with the micrometer at zero and the index set at zero as a slight misalignment. The error disappears with the index set at 90 degrees. It was this error that prompted my first question to Davis Instruments. It was their answer that the "accuracy rate" was plus or minus five degrees that prompted my question to this list. I am interested in the application more than the theory of navigation and piloting. I sail on Lake Michigan in a 24 foot Columbia Challenger and know that it is nearly impossible to take accurate bearings and plot a fix along the coast unless it is near calm. It is interesting to see that the same condition applies to CN. I am new to CN, but it seems that, just like coastal piloting, one is fortunate to determine a position from a series of sights. I think the goal of taking bearings, sun sights, etc. on the small boat is to allow one to compare the apparent actual position to the DR position. I think the final answer to my question about the accuracy of the Davis Instruments Mark 15 is not a numeric specification. Instead, it is the concept that even a plastic sextant that has been made with reasonable care and is handled in a reasonable manner can produce data with errors much smaller than those introduced by the conditions in which the measurement was made. With that said, I want to thank everyone who responded to my original question. George Istok Senior Consultant Signal Hill Consultants, Inc. 574 232 9888