NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Making an artificial horizon, and leveling thereof
From: George Huxtable
Date: 2011 Jan 25, 22:04 -0000
From: George Huxtable
Date: 2011 Jan 25, 22:04 -0000
on Jan 23, Alan provided some observations as follows- ============== Observer's position, GPS coordinates from which I've dropped tenths of a minute was 40 D 34 M North x 80 D 02 M West. No watch error. Index error's as noted. Lower limb shot in all cases. No extremes of either temperature or pressure so far as I remember. Date WT Hs Index Error 19 Aug 2010 16-52-39 72D 42.6M 2.5 M On 19 Aug 2010 11-12-22 99D 18,4M 2.5 M On 9 Aug 2010 09-51-35 74D 58.8M 1.2 M Off 9 Aug 2010 16-16-02 90D 41.2M 1.0 M On 26 July 2010 15-02-17 120D 46.0M 0.6 M On 26 July 2010 18-16-50 50D 30.0M 0.6 M Off 28 July 2010 17-02-15 78D 14.2M 1.0 M Off 28 July 2010 09-51-00 78D 15.8M 0.4 M On 17 Aug 2010 10-23-42 83D 58.4M 2.6 M On 17 Aug 2010 16-01-6 92D 00.9M 1.3 M On Re sights on 26 July, looks like the time interval might be to close. ==================== He ended- "Let me know what, if anything jumps out at you." Before looking at these observations in any detail, one aspect jumps out at me straight away; the variability of his index error, between 2.6' on the arc, and 1.2' off it, including on the same day (August 9th) errors of 1.0' on, and 1.2' off. These seem awful. I don't know what sort of sextant Alan is using, but they are MUCH worse that I get on my cheapo plastic Ebbco. However, I don't own a modern micrometer sextant, though I have used several from other people, so I have no real first-hand experience of what to expect in long-term stability. Is Alan a compulsive "sextant-worrier", perhaps? Does he twiddle at the mirror-adjustment screws, for each set of observations? That's bad practice, but it might explain this apparent instability. Otherwise, it's worth making sure there's no looseness in the positioning of the sextant mirrors. Press on the corners of each mirror, lightly, with something soft and plastic, and see if there's any detectable movement of the reflected image. If the adjusting screws have been slacked back too far, that can leave a wobbly mirror. If it always springs back to the same position, then it's as it should be. George contact George Huxtable, at george{at}hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.