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Author
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Topic: Twilight time in Nautical Almanac
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brokesailor
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posted February 08, 2016 02:00 PM
I am working thru "Hawaii by Sextant" Are the times in the NA for sunset/twilight in GMT? To find my time of sunset/twilight add the arc to conversion to time value?
From: ChesBay
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Capt Steve Miller
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posted February 08, 2016 03:37 PM
The times for sunrise/sunset/twilights are in the 'local' time at the center of each time zone. To find your Local time you would apply your longitude from the center of your time zone (Arc to Time Conversion Table)adding if you are West of the center and subtracting if you are East of the center. To determine the GMT (UTC) of your sunrise/sunset/twilight times apply your Longitude converted to time to your Local times again adding if in West Longitude and subtracting if you are in East Longitude.
Note there are also Tables in the Nautical Almanac on page xxxii to interpolate for your Latitude if that is necessary.
From: Starpath
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brokesailor
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posted February 08, 2016 05:40 PM
If you look at Problem 3B, the answer is 2213.
From the NA for July 7 at N 41 civil twilight would be about 2108 (with DST added in). With DR about 132 W that would be minus 12 minutes for a civil twilight around 2056. What am I doing wrong?
From: ChesBay
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David Burch
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posted February 08, 2016 08:59 PM
For those without the book let me say this is a problem in the text Hawaii by Sextant.
you are given 2 sets of sights. 6 sights at 0827 to 0835 and then 3 sights later from 1400 to 1403.
the job is to average the first 6 to get an LOP around 830 or so (the time you choose does not matter) and then average the next 3 to a sight around 1401. We then combine these two average sights to get a running fix.
to do this averaging and to figure the running fix you need to check the logbook for your DR records between 830 and 1401.
Doing that, you end up with a running fix of
“Problem 3. 1400 FIX using C180, S6.5
41° 37.9’N, 131° 48.7’W”
which is July 7, 1982.
lat = 41 38, on July 7 we get by civil twilight by interpolation 2011, then use arc to time on the Lon to get a correction of 8h 47m, or civil twilight = 2858 UTC, which is (-7h) = 2158 WT .
so i do not see where the 2213 comes from that is in the book. This would not be the precise answer because you are moving, and will not be there 2158-1400 = 7h 58m later, but i cannot imagine that changing the result by 15m but maybe it does. the answer is later, so it means the course must have been to the west.
I will have to ask Steve to please check this. it looks like the course was 176 at 6.4kts (between 1400 and twilight), so both the lat change and the lon change seem in the wrong direction for the time correction we are looking for, ie both corrections make the time earlier than 2158 not later.
so something seems wrong somewhere. maybe i have an error in the above.
From: Starpath, Seattle, WA
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David Burch
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posted February 08, 2016 09:15 PM
PS. daylight saving time (DST) does not enter into anything we are doing in cel nav.
We use two times only, Watch time (WT) and UTC. and in this book the zone description of the watch was +7, ie UTC = WT +7h.
From: Starpath, Seattle, WA
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Capt Steve Miller
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posted February 09, 2016 10:16 AM
I have looked into this issue and I found that I used the median time between Civil and Nautical Twilight to determine the time of 'twilight' sights. Thus the 2213 is correct in this interpretation.
Of course the time 2158 noted by David for Civil Twilight is correct also.
From: Starpath
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brokesailor
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posted February 10, 2016 05:16 PM
Thanks for your answers. I now understand the concept. I do need to reread Dave's blog post on WT/UTC to refresh that concept too!
From: ChesBay
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