Author

Topic: Hawaii By Sextant: Special Problem #1

Ian

posted April 02, 2020 01:44 PM
Hello!
I came within one minute on the Civil Twilight and Sunset, but four minutes away on the Nautical Twilight.
In terms of the process I used, I first take the time of Nautical Twilight for the two nearest Latitudes on the Daily page, interpolate between those two, and then adjust the time based on what I think is called the Zone Time Difference (the difference between the Zone Longitude and the actual Longitude).
In this case:
Twilight at 50N 120W: 02 09 Twilight at 45N 120W: 02 53 Difference 0 44 Proportional Difference 0 09 Twilight at 46N 120W: 02 44 Adjustment for Longitude: 7D x 4 min/degree = 28m Twilight at 46N 127W: 03 12
Was my approach off, or did I make a miscalculation?
Thanks!
From: San Diego, CA


Ian

posted April 02, 2020 05:54 PM
I should note, the info above comes from the Daily Pages of the 1982 almanac
From: San Diego, CA


David Burch

posted April 03, 2020 05:49 AM
For now let me just add that this is for July 5th, 1982, observed from 46N, 127 W and that such times in the book are usually WT with ZD = +7.
For this case the computed answers are:
WT = Times in GMT7 Nautical twilight/Civil twilight/Sunrise 7/5/82 : 04:15:44 7/5/82 : 05:05:22 7/5/82 : 05:43:35 Sunset/Civil twilight/Nautical twilight 7/5/82 : 21:21:07 7/5/82 : 21:59:20 7/5/82 : 22:48:58 LAN/Equation of Time/Hc/Zn rise/Zn set 13:32:27 00:04:27 066°45.5' 055.1° 304.8° LHAA AM Twilight 7/5/82 : 04:40:33 331°17.0' LHAA PM Twilight 7/5/82 : 22:24:09 237°54.7' ====================
The answers in the book are: 1b  Nautical twilight = 04:15:44 = 0416. Civil twilight = 05:05:22 = 0505. Sunrise = 05:43:35 = 0544.
==========
Your interpolation looks right, so 02 44 + 8h 28m (=127º) = GMT, then 7h to get to ZD 7 is 0244 +0128 = 03 72 = 04 12.
So i think the hr is due to ZD 7 (daylight time) but we have to figure out why the tables and computations do not agree exactly. Note that it is generally safest to go to GMT using full DR Lon and then convert back to WT zone.
We will look into this and get back.
Note is is clear that the values we used in the book are the computed ones from the Starpilot shown above. But there is some inconsistency here.
From: Starpath, Seattle, WA


David Burch

posted April 03, 2020 05:55 AM
I see what the problem is. The daily pages include 3 days of almanac data but only one set of sunrise data. That data are for the middle date, in this case July 4th. So to get the right answer to the minute (not needed in practice) you would have to solve for the 4th then solve for the 6th and take the midpoint. Then you have the precise values for the 5th and they should agree exactly with the given answers.
You have brought up a very good point here Thanks for that.
From: Starpath, Seattle, WA


Ian

posted April 03, 2020 07:20 AM
First: I forgot to adjust for DST. I did remember to do that when solving the problem. Thank you for clearing this up: makes enormous sense!
From: San Diego, CA


