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 » Online Classroom   »   » Public Discussion of Cel Nav   » 1B Hawaii by sextant how so exact twilights

Author Topic: 1B Hawaii by sextant how so exact twilights
 navi posted December 15, 2017 04:07 PM                   How do you get such exact values for the twilights?!46N 127W I do arc to time --> 127 degrees west = 8h 28 minI look in the almanac and take the twilights for 45N which is just 1 degree from 46N.ZD is 7 hoursI then get: 8 28 + 2 53 -7 = Nautical Twilight 4:21 WT8 28 + 3 42 -7 = Civil Twilight 5:10 WTYou however get:4:15:44 05:05:22Close enough for practical purposes but how do you get it that exact. From: Chi
 Capt Steve Miller posted December 15, 2017 05:59 PM                   The Naut and Civil Twilight and Sunrise were computed using the StarPilot-89 calculator.Times within a minute are sufficient for our work. From: Starpath
 David Burch posted December 15, 2017 07:11 PM                   Both are right that these do not need to be known to the minute, but that does not distract from the fact that the solutions ought to be identical.Please tell us which problem you are quoting here so this can be tracked down. Done properly, the answers should be the same so one of these is simply wrong... and 415 vs 421 is a big difference.For a moving vessel, there are two steps to predicting the times. From: Starpath, Seattle, WA
 navi posted December 16, 2017 04:36 PM                   Problem is 1B page 11 in the book Hawaii by sextant.For practical purpose I think the precison is good enough for knowing when to get the sextant out. The error ought to be that I use twilight values value for 45N (+ arc to time - ZD as per description in the initial posting) since 45 N is what is closest in the almanac however the actual latidue is 46N.You must use some other method I can't figure out in ordre to get things within seconds. From: Chi
 David Burch posted December 16, 2017 04:58 PM                   The main question might be what is meant by "twilight." Generally i would guess we use halfway between civil and nautical as predicting the sight times.We will check this. From: Starpath, Seattle, WA
 Capt Steve Miller posted December 16, 2017 07:00 PM                   In our normal work we use twilight as half way between nautical and civil twilight.In question 1B the WT of Nautical, Civil Twilight and Sunrise and I stated earlier those were calculated by using the StarPilot-89 calculator, thus the accuracy to the second. Rounded to the nearest minute would be sufficient.Doing the calculations via the Nautical Almanac the times are noted for the Lat lower than the desired Lat and the next higher Lat. and an interpolation is done for the desired Lat. Though this will not give times to the second. From: Starpath
 David Burch posted December 16, 2017 07:03 PM                   Here is the question in the book: “1B. Find the WT of Nautical Twilight, Civil Twilight, and Sunrise for July 5, 1982 at 46N, 127 W. ”And here are the answers from the book: “Problem 1-1b. Nautical twilight = 04:15:44 = 0416.Civil twilight = 05:05:22 = 0505.Sunrise = 05:43:35 = 0544.The answers we give in the book are correct. Please refer to our textbook on cel nav and work a few more of the practice problems on computing times.This is an important process in cel nav, and worth going back to review.also more importantly... we are talking about the motion of the stars. This as well known as anything we deal with in life. We do not want to skip over such a large error and just accept it, without knowing what the discrepancy is due to.. That is bad procedure. There can be rounding errors on interpolation, but it is worth doing it carefully a couple times, then you can round off.A little more practice and you should get the right answer. It does require some interpolations.============================WT = Times in GMT-7Nautical twilight/Civil twilight/Sunrise 7/5/1982 : 04:15:44 7/5/1982 : 05:05:22 7/5/1982 : 05:43:35Sunset/Civil twilight/Nautical twilight 7/5/1982 : 21:21:07 7/5/1982 : 21:59:20 7/5/1982 : 22:48:58LAN/Equation of Time/Hc/Zn rise/Zn set 13:32:27 00:-04:-27 066°45.5' 055.1° 304.8°LHAA AM Twilight 7/5/1982 : 04:40:33 331°17.0'LHAA PM Twilight 7/5/1982 : 22:24:09 237°54.7' From: Starpath, Seattle, WA

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