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Author Topic: eqn of time or mer pass
 navi posted May 24, 2017 05:50 PM                   I am working with excercises 11:12 on page 151,Home Study book.To find WT of LAN this is process used: 12h 00m 00s + eqn of time N lat 12h 00m 00s - eqn of time S lat (I find it puzzling that S lat means - eqn of time or has it to do with what month?)Then WT= UTC-ZDHowever at pages 31-34 mer pass is used instead of 12 +/- eqn of time to find time of LAN.Why these two different methods. Which is best? I do not see any explanation in the book. Also please explain why + or - eqn of time. From: Chi
 David Burch posted May 24, 2017 08:17 PM                   in the text we find UTC of LAN from the mer pass time in the NA.That is given to the whole minute.We also have a note there about finding it to the second using the EqT, but that is rarely needed.This does not depend on the Lat. (your note on that is not correct)it could be in some answers we give it to the second, which would imply we used the EqT method, but again, for normal work of say predicting the time of LAN then the nearest minute is fine. From: Starpath, Seattle, WA
 navi posted May 24, 2017 08:36 PM                   David,I agree with that using mer pass is enough. However for the sake of understanding and clarity I insist on trying to understand the eqn of time method. for excercise 11:12 the answer on page 241 for 11:12 1a) says UTC ( at GW) 12h 00m 00s+ (Equation of time) 05m 23s= 12h 05m 23s for 11:12 2b) it says UTC 12h 00m 00s - (equation of time) 15m 58 s = 11h 44m 02s why + in the first case and - in the second case? From: Chi
 David Burch posted May 24, 2017 09:04 PM                   Fair enough.I will look into what the book says tomorrow.but here is the method. look at the mer pass time and see it if is less than or more than 1200.then look at the EqT given in the NA. this will be some number of minutes (<16) and some seconds.If mer pass is less than 1200, then subtract Eqt from 1200 to get UTC LAN.If mer pass time is >1200, then you add it.That is the practical solution. As i recall there are no signs to EqT given in the NA.I will look at the book to see what we say there., but that is the practical approach.there is indeed a ± to the EqT, but this is a complex function and no simple rules. We have mnemonic tricks for remembering the sign in the Emergency Navigation book.we can also figure the sign EqT from the GHA of sun at 1200 UTC. it will be either before or after 0 or 180. I will look into that and fine tune that point.... however, in this case we will be doing a lot of work for little reward. the EqT will then be that difference converted to time.I will also look up for you a nice article we have on the this somewhere.hope that helps for now. From: Starpath, Seattle, WA
 David Burch posted May 24, 2017 09:24 PM                   Here is an analemma... ie plot of EqT vs. declination. We made this super clear diagram of it for the Emergency Navigation book.See discussion at www.analemma.com From: Starpath, Seattle, WA
 navi posted May 25, 2017 01:34 PM                   Wow, that is quiet complex. It is interesting to know but for practical purposes I will definetely use mer pass rather than eqn of time. Getting the sign right at a rocking boat when seasick and tired is harder than using mer pass. From: Chi

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