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Author
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Topic: Watch Error in Watch Time predictions
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Philip Iannaccone
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posted March 16, 2005 11:20 AM
You ask me to check the printed solution to 21B in Quiz CN03, and I think the answer is wrong or else I am using the watch error incorrectly. Here is my logic.
You asked for the answer to the nearest minute. Arc to time conversion for 121deg 07 min W is 8h 04m 28s, thus LMT is 11h 48m 28s, but the watch error is 4s Fast. So the WT needs to be 4 sec ahead of LMT or 11h 48m 32s or 1149 to the nearest minute. Is that correct?
I love the course materials and I took a lot of time with chapter 3 to be sure I understood what was going on. I haven't done much with the sextant though because the weather is so cold. I did use it a lot last season and as soon as the weather improves I will start with some real readings.
Thanks a lot, this course is just the think for the off season blues!
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David Burch
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posted March 16, 2005 11:27 AM
Hi Philip, thanks for that check and observation. And thanks for your kind words about our materials.
You are right that if a question asks for an answer in Watch Time then we should fold in the WE when we figure the answer.
The solution given to this problem did not do that, so it rounded 48m 28s down to 48m, whereas with the watch error taken into account the answer is 48m 32s which rounds to 49m which would be the correct answer. I will elaborate on that in the solutions section.
Needless to say, in the practical world at sea, one does not ever need these times accurate to the minute. The only place it might arise is when taking a USCG cel nav endorsement exam, and then indeed such details emerge frequently.
Which brings up another point in this particular question, which was to find the WT of LAN. The normal way to do this is to look up the "mer pass" time from the nautical almanac and start from that to correct for longitude and then zone description and then watch error.
The listed mer pass times are given only accurate to the nearest minute. Hence in this case the mer pass time was listed as 11h 44m.
The more precise way to find the time of mer pass is to subtract (or add) the tabulated Equation of Time for 12h from 12h 00m 00s and use that. In this case the eqn of time was 16m 04s, which gives rise to a mer pass time of 11h 43m 56s, which when you complete the problem with these values you get back to 1148, the printed answer!
This not meant as an excuse for our having the wrong answer printed -- i will fix that as stated -- but i just wanted to follow up with some details that we don't normally cover since it is not at all important for practical work.
It is a value of the online course format that we can have these extended discussions on details.
--david
From: Starpath, Seattle, WA
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