GMT Watches and the Nomos Zurich Weltzeit

Edward Hahn

1 February 2014

GMT Watches – How do they Work?
Greenwich Mean Time (GMT) or dual time watches are a fairly common complication in today’s watch world – they allow one to set a second timezone without interrupting the timekeeping, usually with the ability to track your original timezone. They are marketed as useful for world travelers, pilots, and global businessmen – and largely deliver on that promise.

The simplest dual-time watches use two separate movements and dials, which is certainly a pragmatic (if brute-force) method to achieve this goal.

But some would say the classic GMT watch is exemplified by the Rolex GMT Master II, which allows one to adjust the hour hand (and date) both forward and backwards by hour increments, allowing one to read the local time using the “regular” hands. A second, non-quick-adjusting 24 hour hand is provided as reference to hometime, and a rotating bezel allows for the readout of a third timezone (if you can keep them straight in your head.)

Other watches which have this kind of mechanism include the Omega Seamaster GMT movements, which are more sophisticated than the ETA 2893 movements made elsewhere in the Swatch group.

But how does a watch manage to have that indepedently adjustable hour hand in the first place?

The Basics
One of the more basic mechanisms in a watch is the provision of a detent, or method for ensuring that a display is kept aligned with a marker rather than rotating freely. Detents are found on window dates (where they keep the date centered in the window until it’s time to change) and on many chronograph counters (advancing the minute counter one minute at a time.)

A detent is very simple in conception:

This diagram shows a piece of brass (colored yellow) that has been given an edge with a saw-tooth profile. A detent spring (in white) presses down on the saw-tooth, and naturally seeks one of the “valleys” in between two teeth. This mechanism will therefore stay stationary in one position until you poke it left or right. Once it moves one or more slots, the detent spring will again want to fall into a valley.

If you imagine the piece of brass curved into a ring, with the teeth facing inward, that’s basically what a typical date ring looks like. There’s a separate mechanism that gives the date ring a kick one notch over, once a day at midnight.

Now, think about a chronograph minute counter that advances once per minute. Instead of a ring, the mechanism it turned into a gear-like object, with teeth facing outward:

On the top left, you can see the top view of a gear with saw-tooth gears around the outside. As before, there’s a detent spring that causes the gear to want to stay in a set number of positions (12 in this example, but 30 for a typical 30 minute counter.)

The bottom left is a side view that shows how this mechanism can be shaped to carry a hand – in this case, the gear is located on one end of a pipe, to which a hand can be fitted. The right side pictures show a 3D view from the top and bottom angle.

Finally let’s get more complicated and show how a typical GMT movement like the Rolex displays the time. Imagine that instead of having a fixed detent spring, you attach it to another gear, which fits over the sawtooth wheel:

In the top left, we are actually looking at the underside of the assembly. The sawtooth wheel is as before, but the detent spring is now fastened to a new wheel shown in brown.

From the side view (bottom left), one can see that the new wheel nests itself over the sawtooth wheel. The two wheels can still rotate independently of each other (there’s lubrication between them), but the brown wheel will always be in one of 12 positions relative to the yellow wheel.

As before, the right side views show what’s going on in 3D from above and below.

In this case, the adjustable hour hand can be mounted to the brown wheel, which can then be moved into one of twelve detents in the yellow sawtooth wheel. It will also rotate with the yellow sawtooth wheel, which is connected to the timekeeping part of the movement.

(Note: Most versions of this arrangement, including the Rolex and Omega implementation, actually flips this mechanism around – the outer wheel carries the 24 hour hand, and the inner wheel carries the quick adjusting 12 hour hand and detent spring. But the concept is the same. I’ve also omitted some intermediate gearing to simplify the drawing, including the teeth on the edge of the brown wheel, needed to drive the hour hand pipe back and forth. Also, the sawtoothed gears aren’t always shaped that way.)

Which brings us to…

The Nomos Zurich Weltzeit

In 1990, the modern Nomos watch company was founded in January 1990 in Glashutte, the historical watchmaking region of Germany. This in itself is fairly remarkable, as Germany itself didn’t get around to reunification until October of that year.

From the beginning, Nomos has offered a unique set of watches, aligned with the principles of the Bauhaus movement: simple dials and crisp clear printing that facilitate the reading of time on their watches. Initially offered using ETA/Peseux 7001 handwind movements, Nomos has since become a true manufacture, and now only offers watches with in-house movements.

Twenty-four years on, the typical Nomos watch can still be spotted from across the room – a simple white or black dial, stick hands, a simple case with hardly any bezel, and a true cordovan leather strap.

To some extent that has changed with the introduction of the Zurich Weltzeit (“World Time”) and the Xi (ξ) caliber in 2010/2011. While the Tangente GMT version offered a relatively small departure from the canonical Nomos design, the Zurich Weltzeit showed off the design of the movement with a circumferential ring of city names and a disc-based display of home-time at 3 o’clock.

Despite the plethora of dial printing, the watch still manages to carry off the Bauhaus design language.

The dial printing is small, but given the super clean font that Nomos uses, along with the high contrast black on matte silver dial, the watch is quite readable.

The Weltzeit is easy to operate – first, tell the watch what timezone is hometime by pressing the pusher at 2 until your home city is shown at 12 o’clock. Second, sync the hour hand with the hometime (“Heimat”) indicator at 3 o’clock via a corrector at 8 o’clock. Finally, set the correct time.

When traveling, press the pusher at 2 o’clock until a city corresponding to your new timezone appears under the 12 o’clock – the hand will automatically advance (in 1 hour increments) along with your pushes.

Nomos Xi caliber, based on the automatic Epsilon movement. Unfortunately, there’s not much visible to distinguish the two from this side of the movement.

In this case, the worldtime movement uses *two* sets of detents to display and adjust to the time in different cities. The city ring has 24 teeth in the inside surface, corresponding to the 24 time zones that are whole hours from each other.

Unlike the Rolex GMT, the 24 hour indicator is not in the center of the watch, but is relocated to 3 o’clock. Nevertheless, underneath the center of the dial is a sawtooth detent wheel arrangement.

The Zurich watch has a slightly different case from the Tangente, and for Nomos is quite large at 39.9mm. As with the Tangente, the bezel is very thin and the dial is quite large – and as a result the watch has a very substantial presence on the wrist.

The case and substantial lugs are all polished to a mirror finish, and the crown is engraved with the Nomos logo. A stand-off on the case makes the crown especially easy to operate.

The hands are all polished, and the hour and minute hands are faceted. The case is water resistant to 3 atmospheres – but you wouldn’t want to dive with the very nice Horween cordovan leather strap anyway.

Closing Thoughts
As mentioned earlier, there are many different ways to display a second timezone. Patek Philippe’s worldtimers go a step further, by adding a moving 24 hour ring that allows one to see the time in other cities without making any adjustments to the watch. At the other end of the spectrum, mechanisms such as ETA 2893-based watches choose to adjust the 24-hour hand, which presents some issues which I personally dislike.

The path chosen by Nomos makes for a striking departure from their earlier lines, but also has a simple elegance to it. In that sense, maybe it’s not a departure at all.

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