NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Checking a sextant calibration.
From: George Huxtable
Date: 2003 Oct 7, 01:30 +0100
From: George Huxtable
Date: 2003 Oct 7, 01:30 +0100
I agree with much of Fred Hebard's recent posting, but with a few quibbles. >...not all on this list are small >boat navigators. Some, indeed, are BIG ship navigators! Apparently, >they routinely can get their positions to under one mile, and even >under 0.2 miles. I concur with the one-mile figure, but would be reluctant to accept the claim of any big-ship navigator to routinely achieve positions within 0.2 miles. This might be achieved on occasion, however, but only by lucky accident. Anomalous-dip errors, alone, are likely to exceed 0.2 miles. >That is true also of land-based observers. A land-based observer using a good liquid reflector is much better off than the mariner, because his observations don't involve the horizon, and because his errors (with his sextant readings) will be halved. > >I found the quest to determine errors in my sextant to be useful in >motivating me to use the instrument enough to where I became proficient >with it. For me, the key element of proficiency in taking altitude >shots was accurate timing of the sight, while for measuring >interstellar angles, it was accuracy of measurement alone. There are >significant differences between the two. Ultimately, I chose to take >altitude sights since that is the common use of a sextant in navigation >today. For checking sextant calibrations, using the altitude of a body near the meridian can bypass the need for accurate timing. >I don't think errors of 10-30" of arc could be detected by an observer, >but grosser errors, of over a minute, should be easily detectable from >land with an artificial horizon. I certainly agree with Fred's view here.. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================