NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Navigation exercise
From: Frank Reed
Date: 2008 May 24, 04:33 -0400
From: Frank Reed
Date: 2008 May 24, 04:33 -0400
I wrote earlier: "Why do you care to "call noon" at all? This is one of those navigational traditions on commercial vessels (specifically) that people love very much but its navigational significance is hard to fathom." Jeremy, you answered: "I am not sure of this question. [etc.]" Yeah, I wasn't quite sure how to ask it, but your answer gave me enough information, I think, to understand how this usage connects with older tradition. When you say that you "call noon" you're referring ONLY to that moment when you perceive that the Sun has begun to descend. You're all well aware that is some time quite distinct from actual "local apparent noon" but by tradition you "call noon" at that point in the sense that this is when you will stop looking and read off the best altitude. See, historically, they did something else with that call of noon. They set the ship's clocks to 1200. Today, on commercial vessels, you set the clocks to the best local time zone. So the time aboard ship is GMT +/- some integral number of hours chosen in such a way that it's usually not more than half an hour from local mean time. This is the time by which work and meals and so on are scheduled aboard ship (as distinct from the time used in navigational calculations). When you "call noon" today, you could as easily say "mark" or "Sun descending" or something else, but you're continuing a long tradition by using that time-based expression. And you asked: "My question to you Frank, and anyone else who cares to answer, is what methods do you use to observe LAN? I am certainly open to better techniques." First, so there's no misunderstanding, I have never done celestial sights any more than a dozen miles from land. I do navigation education and history. And I don't have the slightest complaint with the way you do the LAN sight. Sounds quite normal to me! I was merely curious about the terminology you use. May I ask, where did you study celestial first? Did you go to one of the maritime academies? Also, you clearly do lots of celestial sights on your vessel. That's unusual these days. I'm sure you do many of them for your own satisfaction and pleasure. Is it also because your captain is a fan of celestial? And you wrote: "Of course in this day in age, it would be better for me to note my GPS longitude, punch it into my computer, get time of LAN, and then shoot it at the exact time to get a more accurate position, even underway." Yeah, and if you've already looked at the GPS output, why shoot the Sun at all (except for pleasure and practice, of course)? And: "The reason it is still practiced, is that at least in the US, the US Coast Guard requires all mates to be tested on this material and even have a practical assessment done (ie go out and shoot a LAN) in order to obtain certain ocean licenses." Do you happen to know which USCG licenses still have a celestial requirement? The situation today reminds me in many ways of the situation with respect to "lunars" in the year 1900. Some licenses required them, but they were almost never used at sea except by enthusiasts. And you wrote: "Historically LAN was shot because it was fairly independent of an exact time piece. LAN was usually advanced or retarded and crossed with an AM sunline advanced to noon." Yes indeed. Well into the middle of the 20th century, on many commercial vessels, even LOPs were dispensed with. Time sights gave longitude and LAN gave latitude. Navigators usually understood that these could be treated as LOP sights but it was the tradition to do a pure longitude sight (a time sight) and a pure latitude sight (a meridian Sun sight). You wrote: "Let's see if my math is correct: The sun is on the meridian at 16h 44m 25s The max HC (70d 09.4') occurs between 16h 43m 30s and 16h 45m 27s at the given latitude. During this time declination changed less the 0.1 minutes of arc. However, since we are steaming south, the sun will continue to rise due to the change in latitude and will peak around 16h 46m at 70d 09.6. The sun will have descended 0.2 minutes of arc by 16h 47m 00s." I haven't checked your math, but I have no doubt that you've got it right. And that fits with my understanding of your use of the phrase "call noon" above. You 'call noon' at max altitude (or shortly after) even though it is not "literally" noon. And you wrote: "Spending a half hour of time shooting a large number of sights to obtain a latitude LOP so that the error is less then a pencil line on a plotting sheet while the captain is breathing down my neck for the noon slip is certainly "horribly impractical" to me." Why don't you just use the GPS then? "On a sailboat it might be different." Sure. It all depends on the circumstances. But I think I should clarify that the purpose of taking a series of sights over 30 or 40 minutes is NOT necessarily to get a perfect latitude LOP but rather to get latitude and longitude simultaneously with only a small amount of calculational effort. You take a series of altitudes, correct them for the distance run and the changing declination of the Sun, and then plot them on graph paper. The peak altitude is worked up as a normal LAN sight (even if you missed the exact time of max altitude, you can get it by graphing). The corresponding time, at the axis of symmetry of the parabola through the points, corrected for equation of time, immediately yields a longitude. This is a procedure that is very easy to teach and requires no sight reduction tables. The resulting longitude is typically about five times worse than the corresponding latitude. You added: "Exactly, and since even a "slow" ship is traveling in some direction(usually changing both latitude and longitude) at 13 knots, and in my case 18 knots 0.3 nm of latitude per minute on N/S tracks), the math involved to correct each of 20 or so sights over 20-30 minutes would require quite a program to correct the changes of latitude and Ho due to the varying position of the ship at each sight." Not, it really doesn't require anything complicated at all. The correction of the altitudes for the vessel's motion and the Sun's changing declination is not hard at all. There are many simple ways to do this. If you have the chance sometime, you might get a kick out of a little navigation manual called "Latitude & Longitude by the Noon Sight" written by Hewitt Schlereth, published in 1982. He had his own particular method for applying the corrections. It was a very simple, easy means of navigating a vessel. I'm not recommending this book as a must-have or the best approach to analyzing this set of sights, but it's nice to see that someone put it down on paper. Jeremy, you also wrote: "The time spent taking that many sights added to data entry and plotting would make a star fix look like child's play." Now I'm certain that you've never tried it out. The method is FAR EASIER than a star fix. But the time involved actually taking sights is definitely an issue. If you're never able to take twelve Sun sights over forty minutes, then you can't do it. Of course, what are you doing wasting time on Sun sights in the first place? You do celestial for fun, right? Your position comes from GPS. "As an aside Frank, I see that you are in Groton, CT." Sort of! I'm in Chicago. That's my primary residence, but I have worked at the Planetarium at Mystic Seaport several times (Mystic Seaport is technically in Stonington, CT but that's literally a stone's throw from Groton, CT), most recently in 2002-2003. I'm in Connecticut quite often though. "I live in Cutchogue, NY; about half the way up the North Fork of LI. It's too bad the conference in Mystic isn't a month later, or I'd be attending. I do hope you do such meetings in the future as I'd like to talk shop." Cutchogue, eh? Very nice. Yeah, it would be great to sit down and talk shop. I'm sorry you can't join us next month. I will probably be in Connecticut again later this summer. I'll let you know my schedule. It's a short ferry ride from the North Fork to New London. I would be happy to show you around Mystic Seaport and introduce you to Don T.; he's been teaching celestial navigation there for nearly forty years. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---