Those Numbers around your Chronograph…
They actually do Something.
By Scott Fahrney
There are certain things in watchmaking that we accept without really questioning them. Dive watches have rotating bezels. Pilot's watches have oversized numerals. Chronographs have a collection of pushers, subdials, and, quite often, a ring of seemingly random numbers running around the outside of the dial.
For many of us, those numbers are simply part of what a chronograph is supposed to look like. They make a watch look fast. They suggest racing cars, pit lanes, leather driving gloves, and perhaps a vintage Porsche disappearing down the Mulsanne Straight.
But those numbers actually do something.
They form a tachymeter, one of the simplest and cleverest mechanical calculations ever incorporated into a wristwatch. And while its most famous home may be the bezel of the Rolex Daytona, Tag Hauer Carrera, Breitling Chronomat, or the Omega Speedmaster. The story of the tachymeter begins long before anyone thought about putting a watch on the Moon.
Before Your Watch Became Jewelry
There was a time when a wristwatch was genuinely a piece of equipment.
Chronographs evolved during the nineteenth century as a way of measuring elapsed time. As the technology improved, watchmakers began adding scales that allowed the chronograph to do more than simply tell you how many seconds had passed. One of those was the tachymeter.
Early tachymetric scales appeared on pocket chronographs and timing instruments before eventually making their way onto wristwatches. By the early twentieth century, the concept had become particularly useful as automobiles, aviation, and competitive motor racing developed. The idea was beautifully simple.
If you knew how far something traveled in a measured distance -- and could measure how long it took to get there, you could calculate its average speed. The watch could do the math for you.
No batteries. No GPS. No digital display. Just gears, a seconds hand, and some carefully positioned numbers.
So How Does It Work?
Despite looking complicated, a tachymeter is almost embarrassingly easy to use.
Imagine you're standing beside a road with two markers exactly one mile apart. A car passes the first marker.
Start the chronograph.
The car reaches the second marker.
Stop it.
Let's say it took exactly 30 seconds.
Look at where the chronograph seconds hand stopped and you'll find it pointing toward 120 on the tachymeter.
The car's average speed was 120 miles per hour.
That's it.
The math hiding behind the bezel is:
3,600 ÷ elapsed seconds = units per hour.
Thirty seconds therefore gives you: 3,600 ÷ 30 = 120.
If the car covers the mile in 45 seconds, the tachymeter reads 80 mph. Twenty seconds? 180 mph.
And here's where it becomes particularly clever. The watch doesn't know what a mile is. It doesn't know what a kilometer is either. It simply calculates units per hour. Time one kilometer and the scale gives you kilometers per hour. Time one mile and it gives you miles per hour. You can even forget about cars entirely.
If a machine produces one part every 20 seconds, the tachymeter will point to 180. At that pace, the machine is producing 180 parts per hour.
It is essentially a tiny analog calculator hiding in plain sight.
Then Came the Speedmaster
In 1957, Omega introduced a watch that would eventually become one of the most recognizable chronographs ever made.
The Speedmaster.
Today, we immediately associate the Speedmaster with NASA, astronauts, and the Moon.
But that wasn't what Omega originally had in mind.
The Speedmaster was born as a sports and racing chronograph.
And one of its defining design decisions was moving the tachymeter away from the dial and placing it on the external bezel.
It was functional. It improved legibility. It also happened to look fantastic.
That black tachymeter bezel became part of the visual DNA of the Speedmaster, and more than half a century later, it remains one of the features that makes a Speedmaster instantly recognizable from across a room.
Of course, history had something considerably more ambitious planned for it than timing race cars.
From The Racetrack To The Moon
During the 1960s, NASA began evaluating wrist chronographs for its astronauts.
The watches were subjected to the kind of testing that makes desk diving seem rather tame: extreme temperatures, shocks, vibration, humidity, decompression, and other abuse intended to determine whether a commercially available wristwatch could survive spaceflight.
The Omega Speedmaster did.
It became NASA's flight qualified chronograph for manned space missions. Then, on July 20, 1969, the Speedmaster became something else entirely.
A Moonwatch.
During the LEM landing at the Sea of Tranquility, the electronic mission timer inside the “Eagle” malfunctioned and died. In short, Commander Armstrong and Aldrin were left without a critical piece of equipment to track their time and coordinate their safe return to orbit. Armstrong unstrapped his watch, started its mechanical chronograph, and hung it inside the Lunar Module Eagle to replace the broken clock.
Omega came to the rescue as Armstrong unstrapped his watch, started its mechanical chronograph, and hung it inside the Lunar Module Eagle to replace the broken clock.
Buzz Aldrin synchronized his watch to Armstrongs, strapped it outside his suit and stepped onto the lunar surface wearing one on his wrist, and the Speedmaster's place in watchmaking history was essentially guaranteed.
But arguably the watch's finest moment came less than a year later. And it didn't happen on the Moon.
It happened aboard a spacecraft that was desperately trying to get home from over 200,000 miles away .
“Houston, We've Had a Problem”
Apollo 13 launched on April 11, 1970.
The mission was supposed to become humanity's third lunar landing.
Astronauts Jim Lovell, Jack Swigert, and Fred Haise were headed for the Moon when, approximately 56 hours into the mission, an oxygen tank exploded. The lunar landing was immediately irrelevant.
Apollo 13 had become a rescue mission.
The explosion crippled the spacecraft's service module and forced the crew to shut down much of the command module to conserve the electrical power they would eventually need for reentry. The lunar module Aquarius became a lifeboat.
NASA engineers on the ground began solving problems that nobody had expected them to encounter.
Among the most important was trajectory.
Apollo 13 needed to hit Earth's atmosphere at the correct angle. Miss the reentry corridor and the consequences could be catastrophic.
The crew needed to make a course correction.
And because of the condition of the spacecraft, they would have to do it manually.
Fourteen Seconds
This is where the story occasionally gets slightly embellished in the watch world. The astronauts did not use the tachymeter to save Apollo 13. They used something much simpler.
The chronograph.
One of the critical manual corrections required the lunar module's engine to be fired for approximately 14 seconds. NASA's historical account records the burn at 14.8 seconds.
The spacecraft had to be manually oriented.
The engine had to be fired. And somebody had to time it. That job fell to the mechanical chronograph strapped to Jack Swigert's wrist.
Think about that for a moment. Apollo 13 was one of the most technologically sophisticated machines humanity had ever built.
It had traveled hundreds of thousands of miles through space.
And when things went catastrophically wrong, one of the tools the crew could still rely upon was fundamentally nineteenth century technology.
A mainspring.
A balance wheel.
A collection of gears and levers.
A seconds hand.
Swigert used his Omega Speedmaster to time the burn – a precise, measured 14 seconds – and the maneuver worked. On April 17, 1970, Apollo 13 splashed down safely in the Pacific Ocean. All three astronauts came home.
Omega was later presented with NASA's Silver Snoopy Award, a special NASA honor given by astronauts to employees and contractors who make outstanding contributions to human flight safety and mission success.
For watch enthusiasts, it is difficult to imagine a better endorsement.
The Tachymeter We Don't Need
Which brings us back to those numbers on the bezel -- The truth is that you probably don't need a tachymeter.
Neither do I.
Your car has a speedometer. Your phone has GPS. If you desperately need to calculate 3,600 divided by 27.4, there is a calculator sitting in your pocket capable of doing it instantly.
For that matter, most of us don't really need a mechanical chronograph either. And perhaps that's precisely why they're so compelling.
Mechanical watches preserve solutions to problems the modern world has largely solved in other ways. A dive bezel remembers a time when elapsed minutes could determine whether a diver had enough air to reach the surface.
A GMT hand remembers an era when crossing multiple time zones in a matter of hours was something remarkable.
And a tachymeter remembers when determining the speed of a racing car required a stopwatch, a measured mile, and a little mathematics.
These functions survive because they connect us to what watches once were – Tools – and the Speedmaster might be the perfect expression of that idea.
It began life as a watch for timing things that moved quickly on Earth. Then humanity strapped it to a spacesuit and took it to the Moon. And when one of those missions went terribly wrong, the astronauts didn't need the tachymeter scale engraved around its bezel.
They needed something even more basic. They needed the watch to do exactly what it had been designed to do.
Keep time.
And for 14 seconds, that was enough.