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» Online Classroom   »   » Public Discussion of Cel Nav   » Celestial Navigation book Exercise 5.8

   
Author Topic: Celestial Navigation book Exercise 5.8
JohnL


 - posted January 11, 2019 08:10 AM      Profile for JohnL           Edit/Delete Post 
I am slowly working through the subject exercise parts and I have a question about how long it should take to fill in a Form 104 worksheet. The first time I did one it took me almost 1 1/2 hours to complete it. I made multiple mistakes including table "parallax" errors and miscalculations. I also had to refer to the text many times for direction. After finishing part #6 I have the time down to about 1/2 hour. This still seems like a long time. Can anyone tell me what is a reasonable time to complete a form? Also, perhaps give me some insight about how this much time would affect practical navigational use while traveling. Thanks.
From: North Carolina
David Burch


 - posted January 11, 2019 12:36 PM      Profile for David Burch           Edit/Delete Post 
Your initial time is about right. It is a learning process. Later on after you have done 20 or 30 of these it will be much faster. Maybe some folks further along in the course could report the time they take to do this. My guess it will be 10 to 15 min. The valuable feature of our form design is this should not be much different for sun, star, moon, or planet.

But you still have a lot to do. In the end, the most time at the nav station when relying solely on your log book and cel nav will be keeping the DR track plot up to date and analyzing sets of sights for the best average. In other words, the time filling in the form will not be the dominate time factor.

You enter the log say every 3 or 4 hours (with course and speed, log and time), then in the late afternoon some time you have to plot out those positions to find your estimated position at the time of evening sights. That could easily take 30 min, done carefully with double checks along the way.

Then when done with those evening sights, we might start an imaginary clock to answer your question.

First it will be another 20 min or so to transfer from your small note pad taken on deck to the navigation logbook the info on your sights. And you have to then update the DR track once again to get the time.

Then if you did choose to just fill out the form for one of the sights, using Nautical Almanac data and the sight reduction tables it might be 10 min at best maybe more like 15… but this is not what you would normally do.

You would ideally have 4 sights each of 3 stars and you would want to figure some average sight that represents the full set of 4, and that is the one your would solve in the work sheet. Here is a note on the full process, but you can cut corners until the navigation is crucial

http://davidburchnavigation.blogspot.com/2018/11/fit-slope-method-to-analyze-sextant. html

Doing this by hand takes about 30 min, but if you are using a calculator or computer it goes very much faster… and indeed our StarPilot Calculator does it all automatically, but we should know how to it by hand before relying on such a tool.

In summary, doing all the analysis by hand takes about an hour to get a fix, and maybe a bit longer when trying to get the best possible fix.

Progress with filling the form will come fast with practice, but we do not want to skip double checking each step.

From: Starpath, Seattle, WA
JohnL


 - posted January 19, 2019 08:53 AM      Profile for JohnL           Edit/Delete Post 
I am still slowly (on again and off again as life activities allows) working through the 5.8 exercise parts so I thank you for your answer. I believe that I have a better feeling that the time I am taking to fill out the work sheet is typical at the level of my skill and understanding.

Though I understand the concepts of curve fitting and triangulation of LOPs, I have to admit that the additional information you gave me in the article is a bit beyond my current grasp of the whole Celestial Navigation process. Perhaps in time it will mean more to me.

Suffer me one more question at the risk of my not understanding the intricacies of an answer. It seems that DR position accuracy is paramount to the process as I understand it thus far. However, as a result of many variables, ones DR position could be quite inaccurate or in the extreme, unknown. Is there a process where your estimated DR position is not required? A yes or no answer would be sufficient. Perhaps, again, some sort of explanation might be helpful.

From: North Carolina
David Burch


 - posted January 19, 2019 10:36 AM      Profile for David Burch           Edit/Delete Post 
Can't quite come up with one word answer, but here is the gist of it.

The DR position is not crucial. If the DR position is wrong then the a-values will be large. In that case, you take the fix you get with the large a-values and call that your new DR position, and then choose new a-Lat and a-Lon and repeat the last part of the sight reduction to get a good fix.

The classic example we use to demo this is to take a fix from the book that has 2 or more sights, and use NYC as the DR position and then follow through as above.

On the other hand, the above discussion, ie DR position is not important, is not the same as saying we do not need accurate DR, considered a verb, not a noun! In other words, you may not know very accurately where you are, but you do need to know what direction you are sailing and how fast your are moving... and this includes estimates of currents and leeway.

If you are not moving at all during the sights, then DR (verb or noun) does not matter at all, but if you are moving, then you must correct the sights for the motion of the boat during the sight session—or more to the point, from one sun sight to the next when doing a running fix.

A running fix from the sun will be just as good as a star fix if your DR is accurate (process not position). If the DR is wrong, then the running fix will be wrong by some amount related to the DR errors, but it is not easy to say how much because this depends on the lay of the LOPs compared to the direction of motion.

In other words, the main answer to your question is the distinction between an accurate DR position (a noun, not needed) and accurate DR as a process (a verb, needed for any running fix).

... and yes, the fit slope method and other special techniques for determining most likely position within a triangle are things that come after these basics.

From: Starpath, Seattle, WA


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