NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Position lines, crossing.
From: George Huxtable
Date: 2006 Dec 16, 00:28 -0000
From: George Huxtable
Date: 2006 Dec 16, 00:28 -0000
Going back to the problem that engaged our attention recently, as stated by John Cole- | Here is the problem that Guy Schwartz asked about and which gave rise | to this very interesting discussion about cocked hats. It is problem | 3-2, p. 22, in "100 Problems in Celestial Navigation" by Leonard Gray | (1999, Celestaire, Inc.). | | 1993 May 14 | 0602 fix 40 31N, 153 08W | C=185 | S=3.6 | I.C. -1.0 | Ht. eye 8 ft. | Alpheratz 14h 28m 17s GMT Hs=36 05.1 | Kochab 14h 29m 52s Hs=42 40.1 | Rasalhague 14h 31m 20s Hs =49 54.2 | Altair 14h 33m 01s Hs=58 56.1 | Venus 14h 37m 04s Hs=16 18.6 | Moon LL 14h 39m 33s Hs=33 15.8 | | Question: What is 14 May 1440 fix? How reliable is it? | Answer: 14 01N, 153 09W. ======================== Earlier, in Navlist 1867, John had written, in relation to Guy Schwartz's posting- "Perhaps you can share your calculations? I worked this problem and my position lines for the Moon, Venus, and Kochab form a very tight triangle symmetrically inside a slightly larger triangle for Altair, Alpheratz, and Raselhague. This gives me 40 03 N, 153 09 W. The book gives the answer 40 01 N, 153 09 W. I calculated my results using Excel spreadsheets of my own design and checked the azimuths using Pub 229." ======================= Comment from George- I have now got round to checking that problem, and my plot rather agrees with John's conclusions. I would guess an effective centre of the six crossing lines to be at about 14 deg 00.5' N, 153 deg 10' W. That's within a mile or so of the dead-reckoning position following the previous fix, which gives one a bit of confidence, and all six position lines pass within a couple of miles of the spot. That plot took into account the shift of the vessel between observations, but as they were closely-spaced enough to cover only 0.7 miles along the track, it would have made little difference if that shift had been neglected. John's results and mine, if correct, imply a serious discrepancy with the plot that Guy Schwartz provided as an attachment to Navlist 1848, on 9 December. It may be wise for Guy to examine his procedures to discover the reason for that disagreement. I will help if I can. Anyway, that plot of Guy's, right or wrong, has provided a welcome opportunity for me to exercise my favourite hobby-horse, about cocked hats. And, it seems, a lesson to a few list members to discover some realities, new to them, about those cocked hats. 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. That is indeed rather different from the conclusion that --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---