Using a GMT and other background info

[petespendthrift]

The GMT-2 is a popular watch but it seems some people don’t understand exactly what it does so here is an explanation (it’s as simplified as possible).

The watch minutes and 24 hour hand are designed to be set to GMT (Greenwich Meridian Time) - now often referred to as UTC (Universal Time Co-Ordinate).

Some centuries ago when astrologists were developing their knowledge of planetary motion and the solar system, Greenwich near London in England was the site of a major observatory. The astrologists there worked out the system of Latitude and Longitude used today that enables us to navigate around the planet by sea and air.

As the planet is a globe, Lines of Latitude called parallels run East/West like horizontal slices through the planet and lines of Longitude (called meridians) run North/South and are like segments of an orange - joining each other at the Poles.

The meridians are numbered in measurements of degrees, with zero degrees being the one that passes through Greenwich, England. The lines then count upwards to 180 degrees if you head either East or West. The meridian at 180 East is the same as that for 180 West - where the counting up meets on the opposite side to the planet from Greenwich (called the Greenwich anti-meridian and used as the international date line).

One degree of Latitude - North or South is equal to 60 nautical miles. One degree East or West at the Equator is also 60 nautical miles but as these lines divide the planet up like orange segments the distance between them gets smaller as you move away from the equator. Near the poles, where all meridians join, 1 degree clearly become no distance at all.

The sun rises in the East and sets in the West. At 1200 GMT the sun is at it's highest point in the sky directly overhead Greenwich.

GMT is the official time at the zero Meridian (Greenwich Meridian).

For every 15 degrees of longitude you travel West the sun will reach the point directly overhead exactly one hour later when compared to 1200 GMT. Likewise for every 15 degrees East that you travel the sun will reach this mid-day point an hour earlier compared to GMT mid-day.

Knowing this actually made it possible to use measurement of the Suns angle in the sky at a given time to calculate where a ship was (in Longitude ie. East/West) so long as the ship had an accurate clock called a chronometer set to GMT.

Time was based on 1200 being the mid-day point wherever you actually were so there were hundreds of different time zones in each country. Later with the advent of trains and travel it became impractical to have a town just a few miles away on a slightly different time zone (making time tabling of the trains extremely complicated!).

All countries eventually adopted what is known as a standardised time system so that whole regions now use the same time zone based upon the regions approximate distance from the Greenwich meridian and generally using changes of whole hours - from GMT (rather than complicated part hour changes). The time zones are named in a way that indicates their difference from GMT which is know as zero or Zulu (eg. +4 or -4 or equivalent letters).

However once aircraft were invented that could cross multiple regional time zones at high speeds the problem was back again and not so easy to solve. Taking off in one zone, then flying through a number of others before landing in yet another different one creates serious problems when co-ordinating with air traffic control etc. So all international aviation uses GMT (now called UTC) to prevent confusion over times. All flight plans are made using GMT.

Once the 24 hour hand and minutes are set to GMT on a Rolex GMT2 they do not need to be adjusted and can be used to tell GMT anywhere in the world. So that the pilot can also have local time and date set on his watch, he simply pulls out the crown to the first position and jumps the (normal) “local” hour hand backwards or forwards as appropriate in one hour intervals. The seconds, minutes and 24 hour hand continue to run, so accuracy is not lost. Moving the “local” hour hand backwards or forwards through midnight jumps the date backwards or forwards appropriately (do not do this on non GMT watches, it breaks them), so the local date can also be shown.

If the pilot would like to temporarily show the time in a third time zone he can rotate the bezel either direction by the appropriate amount of hours of time offset. The 24 hour hand will now point to the third time zone (and can be returned to GMT easily when finished).

For quick reference the bi-colour bezels also shows at a glance whether the third time zone or GMT (whichever is set) is in daylight or darkness giving an quick indication of what part of the day it is in that time zone. Rolex GMT-1 models do not have an independently adjustable “local” hour hand and so have to rely solely on the turning bezel to calculate GMT from local 24 hour time.

The first Rolex GMTs were made on contract for Pan American Airways and issued to their pilots. They subsequently become the watch of choice for both military and commercial pilots.

The popularity of the Rolex GMT amongst pilots has always been that it is a simple and robust watch that provides clearly legible and useful information without any unnecessary extras.

Following NASAs use of the Omega Speedmaster Moon Watch, chronographs also became popular with pilots although they were originally associated with motorsports. Breitling and others have very successfully marketed their chronographs as “real” pilots watches and the general public these days often think that a pilots watch has to be a chronograph.

Most commercial and military pilots will admit that a chronograph is of little use in an aircraft cockpit as a navigation timer as they are generally too hard to read whilst flying due to the small subdials and also not easy to refer to regularly as part of a scan of all the aircraft main instruments whilst worn on the wrist. Aircraft nearly always have a proper timer included in the cockpit instruments. Pilots however love their toys so the chronograph remains popular amongst them.

So now, if you own a Rolex GMT and one day find yourself bragging to your friends in the bar that it’s a pilots watch and they disagree (because it’s not a Breitling Chronograph) you can tell them that it is the original classic and the real deal - not just for posing with - even though that might be exactly what you are using it for!

[brettpaul]

Thanks for the great detail on the GMT! I'm actually looking to purchase one next year and will save your post so I know how to use my watch as I travel internationally for work on a regular basis!!

[MK25toLife]

Very nice story, thanks!

[tictac]

Very instructive,
thank's a lot petespendthrift!

I have just a point to clear about the Rolex GMT Master and pilots.

Some of these watches was given to Air Forces pilots, or civil airlines like PanAm, but IMHO this is just Rolex literature and marketing.

In fact, if a diver finds an interest to dive with a Submariner, as this watch is a tool, a pilot doesn't need a GMT Master.

I am now retired since this summer. I have flown DC-8, DC-10-30, Boeing 747-200, 300 and SP, left seat, captain in command.

We don't need to wear a Rolex GMT or what else watches. We have on bord equipments since the sixties, like inertial navigation systems, based on gyroscopîc devices to cross oceans (INS, IRS systems, and now GPS). We don't care, when we are on the air, about to know what time is it in Honolulu, Singapore or what else, at the same time. We think , eat, sleep and act refering to zulu time. The world wide technical aviation organisation think in zulu time -GMT- We have lot of clocks and chronos on board if we want to play and know what time is it here and there...

This is just to say, the Rolex GMT Master is a great emblematic watch, good for a traveller, explorator or what else, but not really useful for a pilot.

[jnewell]

For a while, I had mine set for UCT/GMT because it's the common time reference in amatuer radio...but sort of like Tictac, I have a dual clock at my operating position so I set the 24 hour hand back to local time.

[SS Oyster]

I just love the fact that you can set it to home time while traveling. Keeping track of multiple time zones is very helpful to businessmen, probably more than pilots.

Have to agree with TicTac on his comments. I retired from the Navy, where everything was in ZULU (GMT) time. We just look at the huge digital clocks in combat or on the bridge. However, when you walk away from your watch (duty ... not the timepiece), it sure in nice to have a timepiece that can remember what time it is back home so when you call, you aren't waking the Mrs. in the middle of the night.

[JimC]

Very nice, informative post. Thanks, petespendthrift!

Good comments by others too. The way I look at it, it isn't always about "function" and sometimes a bit about "style". It can be useful, for the right situation. Or maybe, just to be 'different' when someone notices the extra hour hand.

I'm on the hunt for one now, and looking for what the watch offers as I mentioned above. :)

[tictac]

Originally Posted by JimC

Very nice, informative post. Thanks, petespendthrift!

Good comments by others too. The way I look at it, it isn't always about "function" and sometimes a bit about "style". It can be useful, for the right situation. Or maybe, just to be 'different' when someone notices the extra hour hand.

I'm on the hunt for one now, and looking for what the watch offers as I mentioned above. :)

You are right. I think its really a very useful tool for a business traveller. The old 1675 are a must for collectors, and this 4th hand is a good introduction to the complications watches

[dcmachead]

Excellent, informative post. Thank you for sharing.

[The_Scroop]

Thanks for that, now I can actually explain the function to people when I am telling them about the GMT Rolex's

[jasper]

OK - it's late for me so instead of thinking I'll ask the question.

I have a 16750 GMT-Master - so can not adjust the hour hand indvidually. (stupendously beautiful watch by the way - black bezel and bond nato - wouldn't have it any other way)

*IF* I was an old school pilot who relied only on his GMT to track zulu, then what would I do?

I think (uh oh)

1) While in the air I would leave the watch set to zulu, with the bezel rotated so it's in synch with the 12 hour hand. This way the watch reads zulu both ways.

2) When I arrive in Honolulu I rotate the bezel so that I can read local time off the 24 hour hand. Then I go to the beach/hotel etc, and can kind of keep track of zulu from the 12 hour hand. But if I was confused about 06:00 vs 18:00 for instance I could rotate the bezel back to zulu to tell for sure.

3) When I get back in the cockpit I rotate the bezel back to zulu and I'm off an up out.

Do I have this right?

[Andad]

Item 1. indicates to me that you are setting the hour/minute hand to Zulu and syncing the bezel to the hour hand but the bezel is graduated in 0 - 24 hrs so how do you tell the time on the bezel? IMO you can't keep track of Zulu time using the hour hand and the bezel.

Hang on I just saw item 3. if your a QANTAS pilot then it doesn't matter - we'll just say it's Thursday.

[Nods]

Nice post. As TicTac said the GMT is not much use in a modern cockpit. I do find it useful out of the cockpit to have UTC on my wrist when flight planing and checking weather forecasts. It is also a useful watch when overnighting in other time zones, because of its ease of set. As Pete said in his article GMT changed to UTC. (Universal Time Coordinated) This was because The French did not want to use a standard time zone that was based in England. The deadlock was defused by changing the name from GMT to UTC, although the "0" meridian still runs though Greenwich near London.

[postiff]

This was very informative, thank you.

[Lion]

Thank you for the information and history on GMT's!!!

[Lamone]

Great story!
I love the GMT.

[handdoc]

nice info

[TerryF]

Just to add to this, I read in the manual of the 1675 that it can also be used as a compass using the 24hr hand. Something about using red in the southern hemi and blue north of the equator.

If anyone wants more on it, I'll type it up as it stands in the book. :)

[vaq129e4]

A pilot does not NEED it true, but it is a useful tool. And Pan AM DID order these watches for their pilots, it's not a myth TICTAC. And since Chuck Yeager the ultimate pilot to me has worn one since they came out I think that is a good endorsement.

[MitchSteel]

Its true that Big Tin drivers dont need a GMT watch......cuz everything is automated and there are plenty of references in the cockpit to tell the time(INS,FMS,GPS,cockpit clocks...so on)....However, I find my GMT's very useful when I'm half asleep flight planning across Canadian arctic in the middle of the night for a medevac mission.Some of my coworkes wear Timex, or the Canadian Tire special, some wear Breitling chronos(they're the most made fun of), myself and another skipper both wear GMT and will continue, because they really make our jobs easier.

[Captainz]

As an airline pilot, we use GMT time for everything (releases, dispatch paperwork, weather etc). For ease of conversation, GMT (ZULU) time allows everybody in the airline to function on ONE time, rather multiple time zones..

Simple use for the ROLEX GMT is that it allows pilots to "just" look at their wrists to read the correct ZULU (GMT) time, with-out trying to convert local time to GMT (ZULU) time using HARD BASIC MATH...(i.e it is 700pm in LA what is zulu time? 700pm = 1900 (24hr time)--> 1900 + 7 (time zones to Greenwich or is it 8???)= 0200 zulu-GMT...

I hope this helps.

CZ

[Nightflight]

Peterspendthrift...Thks for your words on GMT2.Did you also put anything out about GMT Master, rather than the GMT 2? Im interested in comments/views about the diffeenrences between the two. Whis is the better watch...from both functional and mecahanical point of view? Which is more collectible? Thanks Nighflight

[Tools]

To add some history perspective to the "original" GMT and it's use.


The use of GMT time was not universal in air traffic in 1954 when the GMT Watch came out, indeed, actual radio/telephone comunications with Air Route Traffic Control Centers wasn't even in place until 1955; so pilots operated on when they would arrive at their destinations using the local time at that destination. Pilots also operated their aircraft based on their home base time and when they would be expected to be back. Air Traffic Control Towers, which pilots didn't contact until they were about 25-50 miles out, were operated locally and didn't come under streamlined/standardized rules until the FAA was born in 1958.. There were also only a half dozen companies even capable of flying across the Atlantic or Pacific.. Threads like this one are always interesting.. but we can't take the way things work today and decide that that's the way it was 50 years ago too...

[gelshocker]

Thanks for posting that.

I can't help but think had the GMT included 24 hour indices -within the dial- it would be perfect. Whenever the bezel is rotated to any other setting other than default, you cannot get a quick-glance reading off the 24-hour hand alone. Which means you can only get 2 quick-glance timezones at any one time.

Which is why there should be a fixed 24-hour track on the dial solely for the 24-hour hand, regardless and independant of the bezel.

Cheers,

[gmtrolexguy]

I am just a little curious about my watch because I purchased it from a reputable dealer in the Jewelry District in downtown Los Angeles. I paid $2800 for my Rolex GMT, it has all the paper work and box but still I am not sure if it is real. I was watching a video on youtube and my watch looks exactly like it and everyone is saying its a fake. Can someone confirm that the watch in the video is a fake?

http://www.youtube.com/watch?v=sYzrzhls_QM

Thanks

[flathead59]

Originally Posted by gmtrolexguy

I am just a little curious about my watch because I purchased it from a reputable dealer in the Jewelry District in downtown Los Angeles. I paid $2800 for my Rolex GMT, it has all the paper work and box but still I am not sure if it is real. I was watching a video on youtube and my watch looks exactly like it and everyone is saying its a fake. Can someone confirm that the watch in the video is a fake?

http://www.youtube.com/watch?v=sYzrzhls_QM

Thanks

I honestly couldn't see the watch in the video well enough to tell. Constantly moving, reflections, etc. If you're concerned about whether or not yours is authentic, take it to an AD and have them open the case. They will be able to tell you definitively one way or the other.

Mark

[petespendthrift]

In the original post I talked briefly about the GMT2 and some general information on the significance/history of Greenwich Mean Time, Latitude and Longitude and timezones. For nostalgia here is an old ad that is fun and interesting.

A 1961 advertisement for the Rolex GMT in which they specifically talk about it's useful features. The photo shows the Sextant (probably on a Boeing 707) that was used through a periscope in the cockpit roof. An accurate GMT time source is essential for using a sextant as every 4 seconds of time keeping error equates to approximately a nautical mile of inaccuracy. Of course navigators and sextants are no longer used on modern airliners (with IRS and GPS) but a well regulated watch and also a GMT reference is still a very useful convenience both on and off the aircraft.

[janice&fred]

Originally Posted by vaq129e4

A pilot does not NEED it true, but it is a useful tool. And Pan AM DID order these watches for their pilots, it's not a myth TICTAC. And since Chuck Yeager the ultimate pilot to me has worn one since they came out I think that is a good endorsement.

don't get me wrong, janice and i own and love plenty of GMT masters, both vintage and modern, but i believe chuck yeager was PAID to wear one.



fred

[SmokieDaBear]

Hello,

I am new to the forums and the brand new owner of a "batman" GMT Master II. I love it. Wear it every day.

Quick question. I thought the GMT hand should be positioned at your local time. For example, I live in EST. When I travel I can put the local time on my watch using the hour and minute hand. That way it is much easier to set. So I have local time and EST on the "GMT" hand.

Does that make sense? For a traveler like myself it is more useful because I can look to see what time it is at home and know what time it is at my location.

[rebel_1]

Originally Posted by SmokieDaBear

Hello,

I am new to the forums and the brand new owner of a "batman" GMT Master II. I love it. Wear it every day.

Quick question. I thought the GMT hand should be positioned at your local time. For example, I live in EST. When I travel I can put the local time on my watch using the hour and minute hand. That way it is much easier to set. So I have local time and EST on the "GMT" hand.

Does that make sense? For a traveler like myself it is more useful because I can look to see what time it is at home and know what time it is at my location.

There is no wrong answer. If your job, life, profession doesn't revolve around the Zulu time zone, then set the GMT hand to wherever helps you the most.