LMT : Local Mean Time

[wholybee]

Originally Posted by Gilletarom

Hello everyone,

Very interesting treatment of the original subject. I learned things.

Should we stop setting up the LMT time in the WeatherRouting plugin? It occurred to me that the creator of the WR plugin, in other words, Sean De Pagnier, is also the creator of the Celestial Navigation (CN) plugin.

And it seems to me, given the content of this thread, that if CN is used then LMT time should be useful. (Not shure ...)

To have ...

Gilletarom.

I don't think there is any reason for LMT for weather routing or planning. Most sailors I know leave the ships clock set to the time zone they left from, and leave it there until arrival at their destination. Some will set it to GMT, and leave it there. And some will update it as they change time zones. But I don't think anyone anywhere uses LMT for anything other than Celestial Navigation calculations.

[P_Dub]

Good Seamanship is to adjust the clock on the time zone as they sail along, that is if you change time zone (in longitude) you adjust your clock during the evening hours (20minutes per watch). That is the normal procedure. The LMT has nothing to do with it.....

[Corsair63]

I've tried to go deeper with the description of different times and really I get not so much information either clarification:

what is really clear is
GMT/UTC: UTC is the evolution of GMT taking a geographical reference in Greenwich observatory and now corrected with the differential seconds.

LT: is the time in the country/area according national rules about time difference with UTC, e.g. most of the Spanish Peninsula is in 0, Galicia is in -1, and government decided in 40s (right before WWII)to be in the same time zone than Germany, so we're in +1 and keeping summer time +2.

LMT: is the corrected UTC time with the geographical zone, e.g. in my case in North Spain with longitude 3W, the time zone is "0" or "Z" (zulu).

what is the result of this? in my case, now:
UTC 1015
LT 1215 (legal time for the country)
LMT 1015

and something we have been talking is corrected Local time for astronomical calculation that is correction of time by converting the arc into time, that will give you the astronomical Local Time of the point, this is only used in some calculations and not needed for anything else.
UTC 1015 and position 003º-46'W, into time 15,1min
Astronomical local time 1015 - 0015 = 1000

complicated, I attach the table of conversion

and it has been told before when sailing and exchanging time zones (LT in open sea you could consider same LMT, but when arriving the coast bear in mind the national rules about time difference) you adjust the time mostly during night time (20 minutes per watch either one officer doing the whole hours and in return voyage same doing the reverse)

[ChrisJHC]

Originally Posted by P_Dub

Good Seamanship is to adjust the clock on the time zone as they sail along, that is if you change time zone (in longitude) you adjust your clock during the evening hours (20minutes per watch). That is the normal procedure. The LMT has nothing to do with it.....

Or you let your smartphone or smartwatch “see” the satellites and it adjusts for local time automagically.

[wholybee]

Originally Posted by P_Dub

Good Seamanship is to adjust the clock on the time zone as they sail along, that is if you change time zone (in longitude) you adjust your clock during the evening hours (20minutes per watch). That is the normal procedure. The LMT has nothing to do with it.....

Are there any writings on this? It isn't what I do, nor a bunch of other sailors. Changing the clock every few days introduces problems. It seems to make much more sense to keep the clock on the same time zone throughout the voyage. Otherwise, one watch will keep losing an hour of sleep, while another gains an hour. And if you are doing celestial navigation, every couple days your adjustment for LT to GMT is going to be different. And during daylight savings there comes the question do you observe it in the middle of the ocean outside of any legally defined daylight savings rule.

So, I keep the ships clock in the same time zone as the port I left from, until I arrive at my destination. But I am open to reasons why not to do that if there are any good ones.

[P_Dub]

It is the normal procedure at sea, if you are crossing many time zone.....

[El Pinguino]

Originally Posted by P_Dub

It is the normal procedure at sea, if you are crossing many time zone.....

Exactly.
Back in the dream time I was employed in the 'Cape Mail'. That was a case where after leaving Southampton the clocks would be left untouched until 2 nights before Cape Town but that was simply so as not to confuse the passos.

That was the only time I have seen that done.

We adjust clocks - either 30 minutes or 1 hour - on my little ship so that the evening meal takes place just before sunset. We don't mess with changing it at night - we just do it around the middle of the day give or take.

Back to the OP and LMT. I don't think LMT has a place in anything at all. LAT is a different matter but even that is pretty much a yesterday thing.
Up until the 1930s some merchant ships would set their deck watches and clocks to 1200 each day at LAN which was of course the time of meridian passage at their location. This is why Norie's Nautical Tables - up until at least the 1960s - had a ' Day's Run - Average Speed Table' covering days of from 22 hours to 26 hours in one minute increments.

LAN is - of course - LMT +/- Equation of Time.

[El Pinguino]

Originally Posted by Corsair63

I've tried to go deeper with the description of different times and really I get not so much information either clarification:

what is really clear is
GMT/UTC: UTC is the evolution of GMT taking a geographical reference in Greenwich observatory and now corrected with the differential seconds.

LT: is the time in the country/area according national rules about time difference with UTC, e.g. most of the Spanish Peninsula is in 0, Galicia is in -1, and government decided in 40s (right before WWII)to be in the same time zone than Germany, so we're in +1 and keeping summer time +2.

LMT: is the corrected UTC time with the geographical zone, e.g. in my case in North Spain with longitude 3W, the time zone is "0" or "Z" (zulu).

what is the result of this? in my case, now:
UTC 1015
LT 1215 (legal time for the country)
LMT 1015

and something we have been talking is corrected Local time for astronomical calculation that is correction of time by converting the arc into time, that will give you the astronomical Local Time of the point, this is only used in some calculations and not needed for anything else.
UTC 1015 and position 003º-46'W, into time 15,1min
Astronomical local time 1015 - 0015 = 1000

complicated, I attach the table of conversion

and it has been told before when sailing and exchanging time zones (LT in open sea you could consider same LMT, but when arriving the coast bear in mind the national rules about time difference) you adjust the time mostly during night time (20 minutes per watch either one officer doing the whole hours and in return voyage same doing the reverse)

Ooooh my brain hurts!!
I think you are over complicating things.
Try this
Basic bit of theory to start.
Page 8 refers https://www.docdroid.net/5rFQH7g/off...ion-pdf#page=8
Earth's orbit around the sun is an ellipse and therefore the length of successive days varies. This means that if you are sitting in your garden on the Greenwich meridian the apparent or true sun would only be on your meridian at 1200Z a few times a year. The mean or imaginary sun would on the other hand be there at 1200 GM(ean)T everyday.

So as you say if you are f'rinstance at 90º West the mean sun will be on your meridian at 1800 Z . However the actual sun will be on your meridian at Long in time ( 90º x 4 = 360 minutes / 60 = 6 hours ) +/- Eq Time.
You can skip most of this by simply lifting the time of mer pas from the daily pages of the almanac and applying your 'long in time' to it. Simples.

So LMT has no real place in the life of the common sailor man.

[Corsair63]

Originally Posted by El Pinguino

Ooooh my brain hurts!!
...
So LMT has no real place in the life of the common sailor man.

Nowadays absolutely agree, for seamen only needed by now UTC & LT.

if you still want to sail like older days that would be your hobby.

still in merchant vessels under CDI either SIGTTO astronomical calculation are required by vetting inspectors and also sextant and cronometre is compulsory to carry on board.
and nautical requirements by IMO(STCW) astronomy is a syllabus of career.

[Corsair63]

Originally Posted by El Pinguino

Exactly.
....
Up until the 1930s some merchant ships would set their deck watches and clocks to 1200 each day at LAN which was of course the time of meridian passage at their location. This is why Norie's Nautical Tables - up until at least the 1960s - had a ' Day's Run - Average Speed Table' covering days of from 22 hours to 26 hours in one minute increments.

LAN is - of course - LMT +/- Equation of Time.

more or less, yes.

still compulsory nowadays in the merchant fleet.

these are requested to carry on board and keep the chronometer logbook with the daily movement of it.

that was because old chronometers were not too much accurate and you must record the daily movement in order to make celestial calculations and more important to fix the longitude.
so at noon time GMT there were quite a lot of shore radio stations that transmit the synchronization signals and the officer of the watch and the sparky tune-up the freq. (a signal consisting in some beeps) and compare with the chronometer on board that was always keep in GMT time, on such way you could predict the error everyday of the clock, that never was adjusted, but recorded the differences in the chronometer log book, just to have a good and reliable movement of it.
use to be set-up in correct time only on every dry-docking when was several days that the error was not recorded.

on arrival of quartz clocks the movements was taken not on every noon and yes every some days because these were most accurate than spring ones.

later TRANSIT system was taken as reference for chronometer.

and by now GNSS is also taken for that.
bearing in mind that GPS time is GPS time not pure UTC, there are some little differences.
GLONASS works on UTC and also Galileo

[P_Dub]

Bear in mind that you must keep your chronometer on GMT (UTC if you like) and being thoroughtly very frequently check. Sun and stars sights need a well adjusted time. This is also needed for keeping your ship's clocks on proper zone time (not LMT).

[Gilletarom]

LMT and time displays in OpenCPN and its plugins....

It's obvious, I suppose, that for most OpenCPN users the choice should be between UTC and local, that is, between UTC and PC time. Bt, this is not the case everywhere.

For example, for the tide display, we have UTC and LMT, but not “Local PC”.