NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunars Tables
From: George Huxtable
Date: 2007 Dec 1, 09:41 -0000
From: George Huxtable
Date: 2007 Dec 1, 09:41 -0000
Scott Owen wrote- | | Bought a copy of Bruce Starks, Tables for Clearing the Lunar Distance a | few weeks ago. From the archives it looks as though Bruce was a regular | contributor but I did not see anything of late. I have two questions: | | 1) Are these the tables most members use for reducing Lunar | observations or is there some other preferred method/tables? [I know | there are many methods to do this, I'm just trying to get a feel for | what method list members are using and what may be considered the most | accurate method.] I've never tried lunars at sea, and these days indulge in such things only very occasionally on land. And I would usually use a pocket calculator, preloaded with algorithms of my own devising, to do the reduction. In my case this would be with an elderly programmable calculator, Casio FX730P or FX 795P, and there is no point, then, in using anything other than an exact clearing mechanism, just as Alex does. I've had a copy of Bruce's tables for many years, and have tried them out in the past, and found that they work really well, and are reasonably easy to use. However, his book is rather bulky for taking to sea. I understand that there was talk of republishing it in reduced-size format, but don't know if that came to anything. The pleasure of using it is in the cleverness with which Bruce has brought everything together to make it all as simple and complete, for the user, as it could possibly be. | 2) Is anyone familiar with the trig/math that Bruce used to derive his | lunar tables? If so, does anyone know if Bruce is amenable to posting | it on the list or has it been posted in the past and I just missed it in | the archives? I did a bit of reverse-engineering myself, years ago, as I was intrigued by how the tables did what they did. I remember that Bruce did, later, put out a posting to the old Nav-L mailing list which explained it. That ought to be in the archives, somewhere. Bruce is still around, very much so, publishing a perceptive and detailed series on Lewis and Clark's journey, serialised in The Navigator's Newsletter. By now it's at part 4, and the expedition hasn't even set off yet! It's giving much detail about their astronomical methods. But I've seen no postings from him here, since the change to Navlist. Some of the tables in the book explain themselves, but others are somewhat mysterious. Here are some of my own deductions, for what they are worth. Scott may like to check them out, numerically. From page 71 to 206 is a table for K, which is a function of an angle (call it A). K = -log(hav A) or log (2/ (1-cosA)) from page 207 to 272 is a table for "Gaussians", actually a table of log ((10^-x) +1) from page 273 to 279 is a table headed log Dec but actually - log (cos dec) from page 291 to 294 is Table 7, which is- log (240' / A) from page 295 to 301 is Table 8, which is log ( 60 / (time in minutes)) ====================== The logs are all (base10), of course. You will note that all are done in such a way that the logs are all of positive quantities greater than 1, so there's no fiddling with negative mantissas. I wouldn't swear to the exactness of those deductions, but they may suffice to put Scott on the right road, as he appears to know what he's doing.. 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 -~----------~----~----~----~------~----~------~--~---