A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Interpolation to latitude
From: Jeremy C
Date: 2009 Nov 13, 15:26 EST
From: Jeremy C
Date: 2009 Nov 13, 15:26 EST
To answer you questions George, some ships keep a clock on the bulkhead reading GMT, including mine. This is fine, but it's a 12 hour clock so it takes a few moments in the extreme time zones to figure out what that clock is reading. Is that 0700 or 1900? Is that today or yesterday or tomorrow? A terrible nuisance to use that clock and I barely glance at it. As an interesting aside, right next to it is a clock set to the ZD of the home office.
The Radars and various GPS equipment register GPS time, which is kept on the Zulu (GMT) zone description. This is what I use most often to figure out what time it is for entry into the date/hour column in the nautical almanac.
My watch, however stays at local zone time, which, incidentally, can be off from the ZD that should be used for our geographic position. There are several reasons for this including Captain's derogative, or perhaps the next port isn't keeping the "correct" time according to their longitude (Ascension Island is a good example of this).
It really doesn't matter too much to me. I note the minute and second on my watch when shooting, and then make sure to note the actual zone description we are using (posted on the bridge) to convert to GMT, or just glance at the GPS to make sure I am not making an error. This is why my examples usually are given in ZT with a ZD included. It is also a dead give away of which twilight it is that would not be as easy to know if I gave straight GMT numbers.
Segueing into another recent thread, the ship not always keeping the "correct" Zone Description is part of the reason I never use DLo to the zone meridian when calculating rising/setting/twilights. I ALWAYS bring the LMT to GMT then back to ZT via -ZD. This not only gives me a better grasp on the entering argument for GHA of Aries, it also cut out one step of the process and circumvents the confusion of knowing what the reference zone meridian is. I always know what ZD it is, and it keeps me from having to remember what the reference meridian is for whatever zone the captain wishes to keep that particular day.
I don't know if it's been mentioned, but I find the easiest way to convert your longitude to time is to use the arc to time page in the NA just before the increments and corrections section. This will certainly get you close enough for Celnav purposes. I convert my whole longitude to time, apply the tabular time to the LMT corrected for Latitude, then apply -ZD (a whole number of hours) and am pretty close. As a disclaimer, this table really only works for the sun, and the moon is a whole other ball of wax.
In a message dated 11/12/2009 7:10:04 P.M. Eastern Standard Time, firstname.lastname@example.org writes:
Joe Schultz wrote-
"...2. The daily pages of a paper nautical almanac are very elegant. Enter
the daily pages with approximate time (left side) or position (right side),
then extract position (left) or time (right) data. Then apply some form of
latitude and longitude corrections to get "exact" position or time. And the
daily pages operate in a zone time world, just like we do in our daily
No. The Nautical Almanac works throughout in GMT, not in Zone Time.
And that leads me to a question, about the practice of American navigators.
What do they use, at sea, to take their time from, for navigating?
Me, I've never been an ocean traveller. My cruising has never taken me out
of the Greenwich time zone. Mostly, it's been done in Summer, when daylight
saving puts us an hour ahead of GMT. If I cross the Channel, local time is
an hour ahead again. But my boat has a bulkhead clock that is always kept at
Greenwich Time, summer and winter, to be used for all navigational purposes.
Do Americans do the same, at sea? Do they keep a clock going, for
navigation, that keeps GMT, and never gets adjusted, running several hours
ahead of zone time? Not changed when the clocks change, spring and autumn;
nor when a time-zone boundary is crossed, and always conformimg with times
that the Almanac uses.
However, my wristwatch does get switched to conform with zone time. I need
to know which timepiece to look at, for which purpose.
I really don't know how many UK sailing people do things the same way as I
do. Presumably, only those interested in celestial matters. There are no
There are some awkwardnesses about doing things that way. You can't use the
bulkhead clock to decide when the news is due on the radio, or when the pub
will close. Nor for working out the tide predictions, which confusingly (and
annoyingly) switch by an hour when the clocks do.
I've noticed that Jeremy's interesting postings contain many references to
Zone Time, so it would be interesting to discover what US merchant vessel
practice is, and exactly what happens when they shift into a different time
zone. I can't say what the practice is in British merchant vessels; does
anyone know? My guess would be that two chartroom clocks are kept running,
one at GMT, one at Zone Time.
Anyway, all that long ramble was for is to point out that to obtain the GMT
of local sunrise and sunset, from the GMT given in the Almanac, just apply
longitude at 15 degrees per hour. And if you want it as Zone time, apply the
appropriate zone correction, in integer hours.
contact George Huxtable, at email@example.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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