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» Online Classroom   »   » Public Discussion of Cel Nav   » Ex. 5.4, No’s. 1.( page 68), 2 (page 72), and 3 (page 72)

Author Topic: Ex. 5.4, No’s. 1.( page 68), 2 (page 72), and 3 (page 72)

 - posted June 10, 2019 09:17 AM      Profile for climber           Edit/Delete Post 
I am reworking this text and noticed something that I haven’t noticed before. In Sample 1 on page 68, the DR Lon is given as 134 degrees, 15min. and the ZD is given as +8. Using the method described on the bottom of page 132 in the text, if I round off the ALon to the nearest degree, 134 degrees, and divide that number by 15 I get 8.93. Rounding that upward, per the instructions on page 132 I get a ZD of +9. Using this method for the other examples in Chapter 5 I get ZD’s that are one greater than the text examples.

This is the case for many of the samples and exercises until I get to Chapter 8. On page 97 the DR Lon is 160 degrees, 25 min. Rounding to 160 degrees and dividing by 15 I get 10.667 which round up to -11 because it is east longitude.

I did not notice this the first time I went through the text because the ZD’s were all given. Why am I not getting the same ZD values as those given up in the text?

Thank you.

From: apalachicola
David Burch

 - posted June 10, 2019 09:23 AM      Profile for David Burch           Edit/Delete Post 
In our teaching, we always refer to the zone description (ZD) of the watch, not of the location of the vessel.

Thus we have


So the standard methods of figuring the ZD to use in Zone time (ZT) do not apply. In USCG exams, they use zone time so you would have


and in this application you could figure ZD by Lon rounded to whole degree divided by 15, then rounded to whole hour.

In our case, ZD of watch, it is what it is, and you cannot relate it to longitude.

also, ZT does not use daylight saving time. On the west coast for ocean sailing to HI in the summer the watch ZD will be 7, whereas the ZT ZD will stay at 8.

Note too that we recommend not changing it. So by the time you get to HI (ZD +10) your watch will be off kilter with the sun by about 3h. Those ships with union requirements to change their clocks, will have advanced the shjips time to +10.

Even in the age of GPS where we know accurate time to the tenth of a second all day long, it remains good practice to not change your ships time when underway. This rules out a lot of opportunity for confusion.

From: Starpath, Seattle, WA

All times are Pacific  
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