NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar trouble, need help
From: Frank Reed
Date: 2008 Jun 26, 07:31 -0400
From: Frank Reed
Date: 2008 Jun 26, 07:31 -0400
George H, you wrote: "Where on Earth does "Tan app. Altitude x 60,53� " come from? For one thing, it's quite the wrong way up, showing refraction increasing with altitude rather than the other way round, and I suspect that must be the result of an accidental slip in his email text, rather than in his calculation. It would be more correct (but for high altitudes only) if the tan were changed to cot, or (which amounts to the same thing) altitude were changed to zenith-angle, and if, at the same time, the constant was changed from 60.53" to 58.293" (see Meeus, also Smart)." Gee, George, give the guy a break. OF COURSE, it was simply a slip in his email text. He meant "tan(zenith distance)". That's obvious. As for the correct refraction constant, his is probably lifted from a historical text. A single arcsecond of difference is not significant to this topic. And you wrote: "Then that amended formula would provide reasonable predictions, for altitudes greater than 20�. But it breaks, down, and badly, at lower altitudes, and then a more complicated expression is required." This is well-known and presumably OBVIOUS for anyone who has looked at the matter for more than two minutes. And you wrote, regarding Kent's account of finding local time: "Well, I have struggled a bit with that, without fully understanding what Kent is doing, which may well be my problem rather than his. It seems a very roundabout procedure." What he is describing is an ordinary time sight (to give local time in order to compare it with the Greenwich Time derived from the lunar observation). The reason his version is so "roundabout" is because he's describing the procedure for getting local time from observations of stars (or other bodies besides the Sun). This is a "textbook" procedure. It's the sort of long and involved calculation that was used to drive students nuts on their exams. It is almost entirely irrelevant to real navigational practice. Real navigators used the Sun for local time and carried it on a watch if they needed it later. And of course the steps required to get local time from a Sun sight are much shorter, and anyone except a complete novice could work them. And I would add that this again highlights the problem of using historical textbooks and navigation manuals to understand the history of navigational practice. Just because the textbook provides a long complicated procedure for dealing with some sight has no bearing on whether that method was used at sea. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---