A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 May 9, 09:53 -0700
I agree with John Howard who wrote:
"I would recomend you have the watch you use set to GMT and use only that time. The date and time for cel-nav is zulu so you can not go wrong."
Just use UT (or GMT or zulu) all the time and forget about zone time. It's cheap and easy in the modern world to carry a watch that always reads UT. The "traditional" (really meaning 'late 20th century') maritime practice of working in zone time is absurd and pointless.
If you must use zone time for problems or an exam, then convert from zone time to UT by adding the zone description to the zone time. To find the ZD, round the longitude to the nearest 15° and then divide by 15, e.g. 71.92 rounds to 75 and dividing by 15 yields 5. In east longitude, the ZD is negative. If your result comes out greater than 24, then subtract 24 and bump up the Greenwich date. If less than zero then add 24 and take back a day for Greenwich. Note that all of this procedural ritual assumes that the vessel is rigorously following the practice of changing time zones at the required longitude for maritime practice. If not, or if you're using civil time (on land), then just ask: what's our offset to GMT?