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 » Online Classroom   »   » Public Discussion of Cel Nav   » Backsight of the Sun Upper Limb

Author Topic: Backsight of the Sun Upper Limb
 Bill Brucato posted January 11, 2004 08:25 AM                   I have a problem with this calculation. Here's the problem:On 6/2/81, because of an indistinct horizon, you take a backsight of the Sun. The hs of the upper limb as observed in the scope is 118*41.4'. The index error is 2.0 on the arc, and the He is 24'. Calculate the Ho.The indicated answer is 61*40.8Now I tried solving this sight this way:first I subtract ie 2.0', then dip -4.8', then 90* from the Hs , then subtract the remainder FROM 90* leaving me with 61*25.4'.At this point it would seem that all I need to do is apply the upper limb correction which for alt 61*25.4' in June is -16.4.The closest I get to the correct answer is 1'greater and that is by ADDING the upper limb correction, what's up?Didn't I orient the sight properly by removing the extra 90* and then using the remainder to find the Hs? And why am I off by 1', I checked the math and can't find the error. Bill From: Celestial Navigation Home/Online
 David Burch posted January 12, 2004 10:52 PM                   Please tell us where this problem originates and who provided the solution you refer to... I will be happy to answer your question, but this background will be helpful since this type of sight is not part of our regular course work... at least i do not recall including it.for those not familiar with this technique, it is in principle used when the horizon in the direction of the body is obscured, but the horizon in the opposite direction is clear. Then if one has a sextant with a large enough arc (many do go to 125 or 130° – to be a "sextant" it has to go to 120) then one can in principle look the opposite way, take this "back sight" measuring from the forward horizon up over your head at 90° then add on the zenith distance to get a back-sight Hs. it is very rare that this would be actually used underway as the body height must fit on the arc (ie above 130-90) and you must have a clear sight to the body backwards as well as to the horizon forwards. Also there could indeed be some issue with arc accuracy at the extreme end of the scale.However, with that said, from an academic and historic point of view, it remains an interesting problem. Please tell us more about where this one comes from and we will have a shot at it.(Also, since this is not part of our course materials, we will after resolving this issue move the discussion to the Cel Nav General Discussion area.)thanks--david From: Starpath, Seattle, WA
 Bill Brucato posted January 13, 2004 09:47 AM                   David, This question is a current USCG exam question no.050177/np00570 published online in the USCG database for deck exams. Also published in the Deck Officer Study Guide Vol. 5 by Capt. J.S. MurphyIt seems a bit ridiculous but I'm certain I've calculated the sight correctly. Have a go at it and let me know how you do.. By the way, is it likely the version of almanac would account for the error? The question is for a 1981 date and I'm using a 2004 almanac for reference.Bill From: Celestial Navigation Home/Online
 David Burch posted January 13, 2004 08:30 PM                   sun and star almanac data essentially repeat every 4 years. 1981 to 2004 is not a multiple of 4 so the data will be off some. ie 1981 = 85=89=93=97=01=05 not 04.you can however, use our perpetual almanc software to compute the data for any year, and since the uscg exams are all for 81, you can buy an 81 almanc several places.also you can get precise data for 1981 from the almanac office online athttp://aa.usno.navy.mil/data/docs/celnavtable.htmlplease check your almanac data with that first, then we will look into the problem. also, what test are you taking? the backsight problems are NOT part of an ocean endorsement to say 100 to 1600 ton license... they would only be on unlimited master according to my reckoning (though i have not kept up on recent changes)--david From: Starpath, Seattle, WA
 Bill Brucato posted January 16, 2004 11:54 AM                   David, I'll be sitting for the Celestial "Endorsement", it's not an upgrade technically. My license will then read Master Steam and Motor/Oceans 1600. I'm practicing celnav problems from Capt. Murphy's text vol.5. I thought it prudent to go over all the material in the celnav section and be as proficient as I can be. I'll try the problem again with the perpetual almanac.Bill From: Celestial Navigation Home/Online
 Bill Brucato posted January 16, 2004 01:39 PM                   I'm sorry David, I suspect the almanac has no bearing on this problem. The corrections for dip and ie do not change. The additional correction for the given date doesn't change enough to account for the fact that ,by following the rules regarding application of the upper limb correction, Ho is 61*09'. The question shows all the signs of being miscalculated by the USCG, the answers given are; 61*19.7, 61*27.8, 61*36.4, and finally 61*40.4'.I don't expect to spend too much more effort on this one.Bill From: Celestial Navigation Home/Online
 Bill Brucato posted February 06, 2004 01:20 PM                   I looked into this problem just a little further and found in "Practical Navigation for the Second Mate" by A. Frost, the following instructions for backsights;"The corrections for ie and dip are applied in the usual manner.By subtracting the apparent altitude from 180*, the correction for semi-diameter refraction, and parallax-in altitude can then be applied in the usual way.(sic)The limb nearest the clear horizon indicates that the upper limb was observed which would appear in the sextant as a lower limb."I did a proper correction to the Hs in the problem and stand by the belief that the creator of the problem added the correction for the upper limb rather than subtracting it. It's the only way he could arrive at 61*40.4. That's all, I just wanted to be sure.Bill From: Celestial Navigation Home/Online
 David Burch posted February 06, 2004 11:02 PM                   Bill, i am sorry i have not got to this yet, but will as soon as i can.... let me please say, however, that in back sights you do indeed do some corrections backwards.... i don't have anything already written up, but will do so shortly... they are tricky to do...depending on upper limb or lower, corrections can be applied backwards (or rather up-side-down, is a better way to put it)--david From: Starpath, Seattle, WA
 David Burch posted February 09, 2004 02:16 AM                   I have added to the Classroom Resources an article called Lesson 11 — Lecture III on Back Sights. I believe that should solve this problem and related ones. Please let me know if there are still any questions about this type of sight.a few graphics will help, but can't do it now.Bill, i must observe that the conclusions in your Feb 6 note above based on an outside reference text do not seem to be correct. Please read through what we have online now to hopefully clarify that. You can test the procedures i have outlined by working several more USCG sun *and* moon back sights.--david From: Starpath, Seattle, WA
 Bill Brucato posted February 09, 2004 01:20 PM                   I'll give them a try. I thank you for your time. I'm also glad that you were willing to take the time and effort to address the problems facing someone heading for the USCG exam room. Your efforts have made this a truly comprehensive curriculum for me.Thanks again,Bill From: Celestial Navigation Home/Online
 David Burch posted June 10, 2008 02:32 PM                   adding a reply to move this to newer date--david From: Starpath, Seattle, WA

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