Everything starts somewhere and Greenwich, England; on the banks of the River Thames, a few minutes from central London and the birthplace of King Henry the Eighth; is the origin for the calculation of time and location in our world.
The Greenwich meridian is the place where 'East meets West' - longitude 0 degrees. Theoretically, noon 'Greenwich Time' is the moment when the Sun crosses the Greenwich meridian. Because of the Earth's inconsistent speed in its elliptical orbit and its axial tilt, this moment can vary by up to sixteen minutes over the course of a year. To establish a standard, a 'fictitious mean sun' was created by averaging the nonuniform motion of the true Sun. For this reason, the word 'Mean' is included in Greenwich Mean Time... so, noon GMT is the moment when this 'fictitious mean sun' crosses the meridian. It's worth mentioning that the difference between the 'physically observed' crossing of the sun on the Greenwich meridian and the artificial crossing of the 'fictitious mean sun' (GMT time Noon) is called the Equation of Time - a neat complication on a few high-end watches that has gained some traction among collectors recently. Martin Braun has done a lot with this and several other 'astronomical complications', particularly interesting is his Notos collection which not only displays the raw equation of time data, but also shows it as a month display and indicates its declination.
As the daily rotation of the Earth is slowing slightly over time, atomic clocks represent a much more stable timebase. On New Years Day 1972, GMT was officially replaced by Coordinated Universal Time (UTC) as the international time reference. Maintained by an ensemble of atomic clocks around the world, UTC is adjusted with leap seconds to keep it within .9 seconds of absolute accuracy consistently. The terms GMT and UTC have since become interchangeable in practical use.
In the watch world, GMT has come to refer to a watch that has a separate hour hand or bezel modification that indicates, or allows you to easily calculate, the time in a second time zone of your choosing. Rolex introduced the first GMT watch in 1955, the GMT-Master was designed to allow Pan Am pilots on transatlantic flights to simultaneously monitor GMT and local time.
You probably aren't too concerned with the antics of the 'fictitious mean sun', but, if you travel a lot, and don't want to risk waking up your kids when you call home during an international 'road trip'; a GMT watch is not a bad thing to have on your wrist.
