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» Online Classroom   »   » Public Discussion of Cel Nav   » Interpolating sunrise and twilight times

Author Topic: Interpolating sunrise and twilight times

 - posted May 08, 2009 10:46 AM      Profile for Roger           Edit/Delete Post 
Can anyone offer insight into how to interpolate sunrise/sunset and twilight times given in the almanac? I have read the instructions many times but I apparently have a mental block. Any help is greatly appreciated. Thanks Roger.
Capt Steve Miller

 - posted May 08, 2009 11:52 AM      Profile for Capt Steve Miller           Edit/Delete Post 
With your DR Latitude (say 27°N) in mind, look at the, for example, the Latitude & Sunset columns. Find the Latitudes that bracket your DR Latitude (that would be 30°N and 20°N). I am looking at the 8 May 2006 Nautical Almanac and I see:
30°N 1842
27°N ????
20°N 1826
Next the difference between the times is 1842-1826 = 16 minutes and the Difference in Latitude is 10°, therefore dividing the 16 min by 10° gives 1.6 min per degree.
Our DR Latitude is 3° from the 30° therefore the 3° is equivalent to 4.8 minutes (rounded to 5 minutes).
By subtracting 5 min from 1842 the time of sunset at our DR Latitude would be 1837.
The twilights and sunrise would be dealt with the same way. Just make sure that you add or subtract correctly based on whether the time is increasing or decreasing in the information in the Almanac.
I hope that this helps.

From: Starpath

 - posted May 08, 2009 02:39 PM      Profile for Roger           Edit/Delete Post 
Fine response. Makes sense and I shall put it to good use. Thanks Roger.

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