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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

 - posted December 15, 2017 04:07 PM      Profile for navi           Edit/Delete Post 
How do you get such exact values for the twilights?!

46N 127W

I do arc to time --> 127 degrees west = 8h 28 min

I look in the almanac and take the twilights for 45N which is just 1 degree from 46N.

ZD is 7 hours

I then get:
8 28 + 2 53 -7 = Nautical Twilight 4:21 WT
8 28 + 3 42 -7 = Civil Twilight 5:10 WT

You however get:

Close enough for practical purposes but how do you get it that exact.

From: Chi
Capt Steve Miller

 - posted December 15, 2017 05:59 PM      Profile for Capt Steve Miller           Edit/Delete Post 
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      Profile for David Burch           Edit/Delete Post 
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

 - posted December 16, 2017 04:36 PM      Profile for navi           Edit/Delete Post 
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      Profile for David Burch           Edit/Delete Post 
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      Profile for Capt Steve Miller           Edit/Delete Post 
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      Profile for David Burch           Edit/Delete Post 
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-7
Nautical twilight/Civil twilight/Sunrise
7/5/1982 : 04:15:44
7/5/1982 : 05:05:22
7/5/1982 : 05:43:35
Sunset/Civil twilight/Nautical twilight
7/5/1982 : 21:21:07
7/5/1982 : 21:59:20
7/5/1982 : 22:48:58
LAN/Equation of Time/Hc/Zn rise/Zn set
LHAA AM Twilight
7/5/1982 : 04:40:33
LHAA PM Twilight
7/5/1982 : 22:24:09

From: Starpath, Seattle, WA

All times are Pacific  
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