LMT : Local Mean Time

[Gilletarom]

Hello Dave,

In the properties page of a route, it is possible to choose UTC, Local@PC or LMT@Location. See screen copy here.

If we switch from UTC to Local@PC, everything is fine. For example, I am at UTC + 2 so a departure time of 6h 20mn 0s will become 8h20mn0s. And vice versa.

BUT
If I choose LMT@Location, how is it that on my PC, under W10, 6h 20mn 0s UTC becomes 6h 09mn 45 s

Where in the PC is the LMT time setting?


Thank by advance.

[houlejm]

Originally Posted by Gilletarom

Hello Dave,

In the properties page of a route, it is possible to choose UTC, Local@PC or LMT@Location. See screen copy here.
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Where in the PC is the LMT time setting?


Thank by advance.

Je ne crois pas qu'il y ait de réglage de LMT dans la PC. LMT est plutôt calculé par O à partir de GMT et de la longitude de ton bateau.
Jean-Marie

[P_Dub]

LMT use the zone time of your computer ( that is daylight saving time if so).
Bye.

[tenchiki]

LMT should be the mean solar time at your location. ie: on the average (hence "mean"), at LMT noon the sun will be on your meridian. The difference between LMT and local time is most apparent when your are at the edge of a time zone, the time zone is in daylight time, or you are in one of the wacky time zones. LMT is really useful in that in PM LMT the sun is to your west, and AM LMT the sun is to your east. LMT is specific to a Lat/Lon location, whereas true solar time is also dependent on the date (since the Earth's orbit is elliptical, causing the actual sun position to lead or lag LMT by some amount, depending on the time of year).


In an answer to "Where in the PC is the LMT time setting?", the answer is "nowhere". The PC clock is a function of UTC and time zone, whereas LMT is a function of UTC and position (for which you need GPS or some other source).



See https://www.timeanddate.com/time/local-mean-time.html

[P_Dub]

Have you ever seen a LMT clock!

[capt jgw]

Back 200 years ago before time zones were adopted everyone set thier clocks to noon being when the sun was highest in the sky. That's Local Apparent Noon upon which LMT is based. If you moved any distance from where you set your clock, the time of LAN would change. So it was like every town had their own time zone which was a little different from the next town down the road. When the railroads came in they couldn't handle that with thier schedules so they invented the time zone. Today LMT is only commonly used in celestial navigation and scientific applications, there are probably apps that can calculate it based on zone time and GPS position, but there's not much usefulness.

[Corsair63]

UTC I guess that it doesn't needs to be explained, it's the GMT astronomically corrected for the differential seconds, briefly.
LMT is the time applied to the geographical area inside the time zones (one zone every 15 degrees in longitude)
Longitude converted into Time is only used for the calculation of position for the observed stars, sun, planet, moon, etc. in order to have the LHA (local horary of Aries) it was and still is used for the calculation of position using the sextant and chronometer using the almanac tables (Browns either the ones issued by the National astronomical Observatories), it was done to know the time of sunrise, sunset, time of meridian and also the correct time for observing (sight) of the stars.
much easy if using the "sight reduction tables for selected stars and sun" because you only need the longitude and not convert arc into time.

I started my Cadetship when still GPS was even not in mind and we had the TRANSIT system, one and if you were lucky two positions a day, so everything has to be done by means of sight reduction and dead recognin.
one start LOPs by sight reduction in morning twilight,
one line of position (LOP) with the sun before the nautical noon
the sun meridian at noon
another sun LOP between noon and evening twilight
one stars LOPs by sight reduction in evening twilight
bearing in mind that that could be overcast sky, so no correction for the dead recognin.
all these calculation carried a lot of practise using the almanac, nautical tables, and on top of it many hand calculations in my time at least we used the calculator, because old seamen still used the logarithm table and everything handcrafter.
of course technical drawing to plot the LOP, at least there was blank latitude charts to carry out at last the graphical calculations.
in merchant vessels there weren't artificial horizon sextants, so no way of stars recognition while night time and not visible horizon.

[wholybee]

Originally Posted by P_Dub

Have you ever seen a LMT clock!

A clock set to LMT, no. But if you are using celestial navigation, you need to calculate the LMT from UTC and your position to solve some problems.

[P_Dub]

LMT as nothing to do with today navigation. I was sailing (probably) much before you and we use GMT on the chronometer, that is all!
So long.

[wholybee]

Originally Posted by P_Dub

LMT as nothing to do with today navigation. I was sailing (probably) much before you and we use GMT on the chronometer, that is all!
So long.

No doubt you have sailed longer than me. Not sure how that is relevant.

Yes, your chronometer is set to GMT because it has to be, because LMT changes as your position changes. It is impossible to set a clock to LMT on a moving ship. But it is not impossible for a computer that knows your position to calculate and display it for you.

A noon sight is taken at noon, LMT. Some people might find an LMT clock useful for stuff like that.

[Corsair63]

Maybe you'll think that is a stupid question, but I've been facing a problem with the plotter and trying to reference to OCPN.

I had the positioning of plotter with auto N2K from my external GNSS and I was "mosqueado" (Spanish) because when I ask for the tide it reffers me to one date in 2022, searching in help of plotter I didn't find any related to manual exchange of date, so I exchanged to internal GPS, and voila the date was updated to correct date today.

just to double check in OCPN I tried to find an option for display the date, but I cannot see any option, neither in the program itself nor in the dashboard plugin.
I can set to see in Dashboard the time but not the date that is using the program, supposed to by the same than the system, but I want to trace the date error in the plotter.

[wholybee]

Originally Posted by Corsair63

Maybe you'll think that is a stupid question, but I've been facing a problem with the plotter and trying to reference to OCPN.

I had the positioning of plotter with auto N2K from my external GNSS and I was "mosqueado" (Spanish) because when I ask for the tide it reffers me to one date in 2022, searching in help of plotter I didn't find any related to manual exchange of date, so I exchanged to internal GPS, and voila the date was updated to correct date today.

just to double check in OCPN I tried to find an option for display the date, but I cannot see any option, neither in the program itself nor in the dashboard plugin.
I can set to see in Dashboard the time but not the date that is using the program, supposed to by the same than the system, but I want to trace the date error in the plotter.

It is probably best to start a new topic. But, I'm not sure if OCPN decodes the date. Best would be to record the N2K data and look at it there. If you have a N2K to NMEA0183 converter you use to connect to OCPN, most of those have that ability.

[P_Dub]

Corsair, the question is not stupid, for that you will have to refer to Dave. Send him you log file of the error. Someone may find the problem, if is not a computer adjustment time.

[rgleason]

I think, to improve the function of planning, it would be really nice, to have a way to change the date and time (also selecting LMT, UTC etc) in OpenCPN itself. I think this improvement would lead the way to OpenCPN having better planning features.

Perhaps this feature could also be integrated with Grib_pi and otcurrrents_pi which already handle date and time (API change needed?).

[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.