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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Lon. by LAN

   
Author Topic: Lon. by LAN
billstrath


 - posted January 17, 2011 08:00 AM      Profile for billstrath           Edit/Delete Post 
In prep for a trip I'm reviewing all the materials gathered when I took the course. Having read the article in the course book on Lon by LAN I forgot about the method. However, in reviewing the User's Guide on a Davis 15 Sextant, it appeared to outline a simple method. I was wondering what your opinion of it was. Its on page 11. If you don't have the user's guide, its basically simple. After having taken several Hs' and as the last Hs begins to decsend set the sextant to the height first of the previous ascending Hs, continuing shooting the sun until the sun on the horizon with the predetermined setting. Note the time. Average that time with the time of the Hs you used to preset the sextant for the last sight. This would be your longitude after converting.
From: Lakewood, CO
David Burch


 - posted January 17, 2011 09:03 AM      Profile for David Burch           Edit/Delete Post 
Yes, you can do that if you are not moving. If you are moving you have the problems addressed in the article.

However, as we have said numerous places in the materials, if you know how to do a sight reduction, you are much better off just taking several sun lines and plot their intersection. it will be faster, and more accurate.

Lon by LAN is a back up method that assumes you are missing some component of your regular gear. It is thus a featured method in our Emergency Navigation book, and not covered at all in our regular navigation training program, which assumes you have what you need.

I might even go further to say that when you do a routine LAN sight, you inevitably end up with a time for the peak height as you need this to look up the declination.

At that stage of the process you are tempted to look up the GHA of the sun as well and think about that as a longitude.

and my advice and experience with this is to deny that temptation. it can easily mislead you into questioning what is likely better knowledge of your longitude from other sources.

put another way, if you want your longitude, then simply sight reduce a few of the sights from before and after LAN and get a fix.

then if you ever lose your sight reduction tables, or almanac, or sextant or watch, then you can come back to this Lon by LAN method and be pleased with the results you get under those conditions.

From: Starpath, Seattle, WA


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