On May 2, 1952, 36 people boarded a De Havilland Comet jet owned by British Overseas Airways Corporation (BOAC) and flew nonstop from London to Johannesburg. The Jet Age had officially begun. Pan Am quickly ordered 20 of Boeing‘s new 707 jetliner and built its WorldPort terminal at JFK while TWA countered with its own World Flight Center. It was a brave new world that saw cigarettes on planes but not a roller bag to be found. People were suddenly able to fly across oceans and time zones in a day, which also introduced the decidedly modern ailment known as jet lag.
The pilots flying these long haul routes needed a way to keep track of time, both local time and the time in the place from where they departed. Pan Am approached Rolex, the watch brand that Hillary wore up Everest and Cousteau wore in the deep, and requested a watch built for this new breed of pilots. In 1954, the GMT-Master was born, a watch that could track two time zones simultaneously.
The GMT watch is brilliant, not because of its complexity but because of its simple ingenuity. Because the going train in a conventional watch is geared to spin the hour hand around the dial every 12 hours, all that is required to become a GMT tracker is simply a second hour hand geared to run half as fast and an additional 24-hour time scale. In other words, the 24-hour hand circles the dial once a day instead of twice, its tip pointing to the corresponding hour, leaving no confusion as to whether it’s a.m. or p.m.
While GMT watches were first created for pilots and are historically linked to aviation, they have since taken on a reputation as a consummate adventurer’s watch, useful for the one-watch traveler who wants to touch down in Kinshasa and head straight into the jungle. Good water resistance, rugged cases and legibility are the hallmarks of the new breed of GMT watches.
More explanation :
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.
Source : https://gearpatrol.com/2015/03/17/complication-series-gmt/ , https://www.rolexforums.com/showthread.php?t=55173
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