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» Online Classroom   » Emergency Navigation   » Public Discussion of Emergency Navigation   » Finding the date with only UTC time, sextant, books

   
Author Topic: Finding the date with only UTC time, sextant, books
JohnM


 - posted November 09, 2020 11:58 AM      Profile for JohnM           Edit/Delete Post 
In the movie "Adrift" (book RED SKY in MOURNING) the author has a watch and clock for time, sextant, and I assume the necessary tables needed for navigation.
All electronic equipment (radio, GPS etc) was dead. The movie and book don't explain how she found the date or day. From the log book she assumed she was only unconscious for three hours therefore she chose the day based on this assumption. However, it turns out later that the Coast Guard determined that she must have been unconscious for at least 22 hours. She would have been off in her calculations by a full day. This seems like a huge error but perhaps she corrected it somehow. Could someone explain how she might have done this?

From: United States
David Burch


 - posted November 09, 2020 12:32 PM      Profile for David Burch           Edit/Delete Post 
With a watch, sextant, and tables, you can find your position using normal cel nav.

If for some reason your watch did not tell you the date, you can simply look at moon in the stars to tell the date. This can usually be done by inspection, leaving the sextant in the box. The moon moves about 12º per night (half an outstretched hand width) relative to the stars.

As for DR error in 22 hr "adrift" in a sailboat, that is not a large factor at all. Depends on wind and current but even sailing at full speed all trimmed up it would not likely be more than 100 nmi.... but again, this would not matter if all the equipment you mention were actually present.

This would be a good exercise: next time you see the moon at night in the stars. Just sketch its relative position to some prominent star, then do a couple sight reductions for Hc and Zn of the two bodies, or much faster, use the program stellarium to see how long it takes you to figure out the date.

From: Starpath, Seattle, WA
JohnM


 - posted November 10, 2020 01:30 PM      Profile for JohnM           Edit/Delete Post 
I found that Star Pilot, the celestial navigation program, has a window called Sight Planner that has a sky plot. The sky plot does an excellent job of looking at the moon and star relationships as well as time.
From: United States


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