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» Online Classroom   »   » Public Discussion of Cel Nav   » Time of Sunset While Running

   
Author Topic: Time of Sunset While Running
michael gondro


 - posted November 04, 2004 11:19 AM      Profile for michael gondro           Edit/Delete Post 
Dec 10 1981,your 1300 ZT DR is LAT 26*27.0 S
Long 79*04.0 E course 068T Speed 14kts. What will
be the zone time of sunset at your vessel.

David Burch


 - posted November 05, 2004 09:47 AM      Profile for David Burch           Edit/Delete Post 
This appears to be a USCG exam question. We are happy to look into solutions of this type, but since they are mostly of value for license exam prep, it would be valuable to include the proposed answers and the actual question number if known. Please post those and we will make an outline of the solution that can be applied to related problems.

Note these USCG questions will all be from 1981. You can get the almanac data online from our INET resources section under celestial, or use our Starpath Perpetual Almanac, which works backwards to about 1580 or so. There are also other sources of historic almanac data online. Also there are reprints of the full 1981 almanac available for sale, but they are generally rather expensive. Check with Marine Education Textbooks in Houma, LA if you want one of those.

Again, please add the answers given and the question number if known. With the spread of answers given, the student has a better feel for the precsion needed as well as a nice illustration of the typical errors.

Note that the USCG cel nav questions are not "tricky" for the most part, but the wrong answers are not random. Each wrong answer represents a typical mistake, so one must be very careful in gaining confidence just because you see your particular answer on the list. In this sense, the wrong answers are traps.

From: Starpath, Seattle, WA
michael gondro


 - posted November 05, 2004 10:58 AM      Profile for michael gondro           Edit/Delete Post 
The question number is #1267 and the ansers were
1824-1846-1854-1908. I have had some success with
a few other questions but need a better way of
dealing with the speed and distance traveled during the time it takes for the sun to set or
rise. Thank you for the help. Mike

David Burch


 - posted November 05, 2004 12:38 PM      Profile for David Burch           Edit/Delete Post 
This problem is worked in two steps. first find out event time when not moving as a first estimate, then DR from present time to that time to get a better estimate of where you will be at that time, then do the estimate again using the better DR position. In the real world this extra step is rarely called for since we are not often moving fast enough to change the time much, but for a ship at 15 or 20 kts this can shift the time several minutes.

First, lets have a look with the StarPilot calculator or software program. We might be off as much as a minute, but not more.

Procedure:
(1) find zone time of sunset assumed S=0 from DR position. figure ZD as Lon rounded to nearest degree/15 and then round to nearest whole number. ie 79/15=5.2666 ---->5 and in east lon this is -5h

So use settings to set UT offset to -5h, then find sunset at DR position to be 18h 29m 18s.

that would be the answer if we were not moving at all. often this will be one of the choices, it was not in this case.

(2) between present time of 1300 and time of sunset 1829 we will move 76.84 miles in direction 068. this you can solve by plotting or better by computation. with starpilot go to setting and make DR mode = speed, set DR time to 1300 and enter course and speed... then from main menu do update DR to 18 29 18 to get the new position which is 25° 58.2' S, 80° 23.0' E.

(3) now look up the sun set time again from this position and get 18 22 59 which is 1823, so the first answer 1824 must be right.

this is off a minute from the answer, but i do not know without working with the actual 1981 almanac data which is correct.

we will add some more details on the solutions by hand with paper almanac as soon as we get them scanned from our notes.

--david

From: Starpath, Seattle, WA
michael gondro


 - posted November 05, 2004 03:59 PM      Profile for michael gondro           Edit/Delete Post 
I used the NA arc of time table and the interpolation table in the back of the alminac
and got grear results. The only one that gave me a
hard time were the ones where the vessels speed and course were east or west. when it was north or
south the time were right on. mike



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