NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Celestial Navigation without a sextant.
From: George Huxtable
Date: 2008 Mar 10, 20:32 -0000
From: George Huxtable
Date: 2008 Mar 10, 20:32 -0000
Greg Rudzinski wrote- | In a lifeboat with a chronometer,tables, and compass (no | sextant), a navigator would be able to mark the GMT time of the sun's | limb on the horizon. The elevation is zero. Correct for refraction, | semi-diameter, and height of eye (dip). Perform normal sight reduction | and expect 6 minutes of arc accuracy under normal weather conditions. What does Greg base his "6 minutes of arc" figure on, when it must include the uncertainties in refraction for the Sun, as seen on the horizon? How well does he know what that refraction will be, and how much it might vary from the "book" value of 34 arc-minutes? I quote from Meeus, "Astronomical Algorithms", chapter 16 on Atmospheric refraction (page 107 in my 1998 ed.). " ... According to Schaefer and Liller, the refraction at the horizon fluctuates by 0.3 degrees around a mean value normally, and in some cases apparently much more" The reference is to "Refraction near the horizon", Publ. astron. society of the Pacific, vol 102, pages 796 - 805 (July 1990). It would be nice to see more detail, but I don't have access to that original paper. Brad Schaefer is a recognised authority on optics in the atmosphere, and more recently has produced an authoritative series of papers titled "Sunset Science". However, there are very divergent views on this matter. My 1977 edition of Bowditch states, in vol 1 page 422 (para. 1606)- "Generally, the error in tabulated refraction should not exceed two or three minutes, even at the horizon". I regard that statement as absurd, but you can take your pick between those views. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---
