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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Finding Longitude

   
Author Topic: Finding Longitude
Craig001


 - posted October 21, 2014 07:54 PM      Profile for Craig001           Edit/Delete Post 
When I was taught CelNav I was told if you divide your DR longitude by 15 it gives you the ZD. In Hawaii by Sextant that doen't seem to be the case, was I taught incorrectly, or am I missing something
David Burch


 - posted October 21, 2014 08:22 PM      Profile for David Burch           Edit/Delete Post 
To find the ZD that is used in Zone Time, you round the DR longitude to the nearest degree, then divide by 15 and round to nearest whole hour.

However, we do not use zone time in the course, nor underway. We use Watch Time, and watch time uses the ZD of the watch, not the ZD of where the boat is located.

For that voyage, and for every other summertime voyage across the Pacific leaving from the West Coast we have done, we use zone 7. Then when we get to Hawaii we switch to zone 10.

Generally on the way home, we set the navigation watches to zone 7 on leaving and arrive on time.

The key point we stress in the course is to NOT change your watch time when underway. The practice of doing so is primarily to meet the needs of labor unions for folks who work at sea. It only confuses the navigation and opens new roads to error.

With that said, I should add that USCG cel nav questions do indeed use ZT, which is figured as outlined above.... and even worse than that, they give the sight times in terms of CT (Chronometer Time), which is UTC on a 12-hr watch face —without specifying if it is AM or PM. Thus almost every USCG cel nav test question starts out with the challenge of figuring out what time it is.

From: Starpath, Seattle, WA


All times are Pacific  
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