A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The cocked hat
From: Joel Jacobs
Date: 2004 Apr 5, 13:20 -0400
From: Joel Jacobs
Date: 2004 Apr 5, 13:20 -0400
Robert, Just do whatever your comfortable with, but I recommend you try and use an odd rather than an even number of sights. Nautical Twilight at sea where you are not inhibited by landmass in any direction is long enough to give you adequate time for a round of sites. It's length varies with your lat and the season of the year. I don't have an almanac, but if you do, take a look at what happens as you go from the equator N. The higher the lat, the longer the twilight. The range of useable twilight may be as short at 30 min and as long as 2 hours. Even much longer at the poles depending on the season. The bottom line is that is rare to not have enough time. Glad to hear your mastering the CPU 300, Joel ----- Original Message ----- From: "Robert Eno"
To: Sent: Monday, April 05, 2004 12:44 PM Subject: Re: The cocked hat > Thanks Joel, > > I see your point vis a vis the relative consistency of the bubble horizon, > however I should point out that it is not as consistent as you might > believe; in fact it is very fickle at the best of times. Even on land, when > taking bubble observations, the average error is likely to be in the realm > of 2 minutes of arc or more. Anything less is fortuitous and icing on the > cake. Now consider taking these observations in a moving aircraft. It is > painful. A bubble at sea is, in my experience, of dubious value so I won't > discuss it. > > I agree that the horizon at sea is variable (how could I disagree?) and that > Greenaway did not take this into account because, after all, he was talking > about air navigation. Nevertheless, I still see the value of the two times > two star shots if you can pull them off. But now you've got me to thinking > (I am not wholly obstinate and set in my ways) when you discussed -- for > perfectly valid and sensible reasons -- taking three shots instead of two. > In this, I also agree with you. In fact I like to try for four shots of each > (I get carried away when I get a good opportunity for a star shot and a > perfect sea horizon is for me, the same as a red flag for a bull, except I > grab my sextant instead of charging at it). > > Nevertheless, I more or less subscribe to the idea of taking full advantage > of the limited time afforded by that magic period of time where star and > horizon are visible simultaneously, by concentrating on just two stars > rather than spreading myself out too thin. > > Having said all of this, I have to acknowledge two things: > > 1) The bulk of my experience in astro-nav, pertains to land-based > operations, however, in my own defence, I have used this technique at sea > and successfully. > > 2) I acknowledge that Joel and many others on this list have a level of > experience and knowledge that vastly outclasses my own, so I am not arguing > against you but simply expressing what works for me. > > Oh yes, Joel, I am getting more proficient at that star finder. It is a very > neat piece of gear once one gets the hang of it. > > Robert > > > > ----- Original Message ----- > From: "Joel Jacobs" > To: > Sent: Sunday, April 04, 2004 9:37 AM > Subject: Re: The cocked hat > > > > This is in response to Robert Enro. > > > > I have a few problems with Robert's recommending RCAF Navigator's Keith > > Greenway's two sight, two average fix method for use at sea. > > > > IMO, it is subject to the following deficiencies. Greenway is taking > sights > > from an aircraft using a bubble sextant so that the instrument provides > the > > same (bubble) horizon no matter in which direction he is shooting. You > don't > > have that at sea when using a traditional sextant. As everyone knows, the > > horizon at sea's definition varies with the direction you are looking due > to > > differences in light, sea conditions and cloud cover. Therefore, you have > > variables with which Greenway is not confronted since his horizon is a > > constant. > > > > The other issue is taking only two sights. I think that's a bad idea > because > > an error in one can only be halved when combined with the other. In most > > things, where you are trying to have a definitive outcome it is always > wiser > > to use an odd number so that you don't have an even split where one group > > can cancel the other out. I don't understand how Greenway concludes that > an > > error in a two sight fix is obvious. IMO, that it is impossible to judge > > which of the two parallel sights is in error. So what do you do? He > suggests > > that you go back and shoot it over again. That is not always possible, an > if > > it is, because of the significant time lapse, you have a running fix, not > a > > fix. All something to think about. > > > > Robert, with that nifty and very unusual star finder we discussed off > group, > > I suggest you predetermine the alt and az of the bodies beforehand. Set > you > > sextant's to same (adjust for IC, REF, HE) accordingly, then adjust for > > variation and deviation, and note the adjusted compass bearing to the body > > for each object, and just aim along that line. You should have the target > in > > view. Some minor adjustment's and you have your sight, > > > > I preferred taking a minimum of three near simultaneous sights of the same > > body, and then moved on to the next. The planning took into account > > diminishing or increasing light conditions, and where I had to position > > myself for steadiness etc., as I moved from one target to the next. I > based > > this on where I would have the best horizon. If possible, I always used at > > least three bodies. Two sight fixes would more often be shooting an early > or > > late rising planet or moon and crossing it with the sun. Averaging was > done > > with a calculator that had the ability to manage time using Sexagsimals, > > base 60 numbering system. > > > > Let me add, this was more a matter of self-entertainment, because as I've > > said before, when you're out in the middle of the ocean, with no hazards > at > > hand, all that precision is unnecessary. But it was fun. > > > > Joel Jacobs > >