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» Online Classroom   »   » Public Discussion of Cel Nav   » Ex-Meridian Passage/Star

   
Author Topic: Ex-Meridian Passage/Star
Bill Brucato


 - posted March 05, 2004 09:30 AM      Profile for Bill Brucato           Edit/Delete Post 
Hi David, I've been working through my Exam Prep text and came across an ex-meridian problem that uses a star as opposed to a body. I can successfully render my answers with regard to ex-meridian observations re; sun, moon, or planet, but I'm stumped on how to do this with a star. How do I determine the mer-pass of a star?
Bill Brucato

From: Celestial Navigation Home/Online
David Burch


 - posted March 06, 2004 11:08 PM      Profile for David Burch           Edit/Delete Post 
Bill, first the foremost point. i can almost certainly assure you there are no ex-meridian sights of any kind on a 1600 ton cel nav endorsement test, let alone of a star.

nevertheless:

(1) a quick way to get the mer pass time of any star is to use its SHA and then refer to the planet diagram, and interpolate as needed.

(2) more precisely, look up the mer pass of aries -- given on the daily pages -- then add the SHA of the star (always west of aries) converted to time with the arc to time table. Then correct for your longitude to find the time of observation.

Note you must be careful looking north that you are not dealing with a circumpolar star that has both an upper transit (east to west over the top of the pole) and a lower transit (west to east, beneath the pole). Either can be used, in principle, for mer passage of a star.

(See topic called ex meridian sights in this forum for more info on the overall procedure.)

Note that though this is not part of routine navigation -- nor even USCG testing any longer, for that matter -- it is covered in my Emergency Navigation book, since the altitudes of low stars (lower transits) can be measured within a few tenths of a degree with a 15-cm ruler. In this sense alone, it is a novel but perhaps one day useful thing to know.

once the crunch of your test prep is over, you might want to look into that book for some fun applications of your cel nav knowledge.

--david

From: Starpath, Seattle, WA
Bill Brucato


 - posted March 07, 2004 07:09 AM      Profile for Bill Brucato           Edit/Delete Post 
Wow, Thanks again David. I indeed found that I am not required these skills for the exam. As I was working through the study materials, I got hung up (and a bit determined to master as much of this material as I could)and couldn't get past it without finding out how it worked.
I did find a short article in an old edition of Bowditch that addressed ex-meridian passage, and I was able to complete the problems using the sun. I ran up against it when I was faced with a star.
My curiousity is satisfied in this regard.
Thank you so very much, I feel that I am prepared for my exams and I will let you know once I have completed taking them. You're help has been invaluable.
Bill Brucato

From: Celestial Navigation Home/Online


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