People have known for centuries that watches can do more than tell time without adding special gizmos. For example, you can tell which general direction you’re facing if you point the small hand at the sun, and then look down the line between the the small hand and Noon. That line points to South if you’re in the northern hemisphere. This works a bit different in the southern hemisphere.
Some sport watches, usually for men, have extra numbers around the face, sometimes on a moving bezel. If you look closely, you’ll see the word “tachymeter” written in tiny letters. What’s it for?
I had one of these watches when I was a kid, and I figured it was similar to a tachometer, but I had no idea how it would tell me how fast the engine was revving in a car. In fact, it’s more useful with horses.
A tachymeter does exactly on thing: it divides the number 3600 by the number of seconds that have elapsed. While that may sound arbitrary, it’s not. 3600 is the number of seconds in an hour. So long as you have a measured distance, you can learn the average speed of an object traveling between point A and point B.
In the real world, it might look like this: you’re watching a horse race and it’s a 1/2 mile long. Before the race, you stop your watch so the second hand is at Noon. When you hear the gun, you start your watch. Your horse is blazing the track, and wins by a full length! You stop your watch at the exact moment he crosses the finish line. 38 seconds have elapsed, and the tachymeter reads 94. Your horse ran at a pace of 94 half miles an hour, or 37 miles per hour. That’s as fast as Secreteriat!
While tachymeters do work, they have some severe limitations. Stopping your watch at Noon is tedious, which is why the fancier watches have rotating bezels that allow you to start wherever the second hand is. But what if you’re measured distance takes longer than sixty seconds to complete? Then, you’re out of luck.
If your watch has a tachymeter, now you know what it’s for. But have no illusion that watchmakers expect you to use this feature—it’s really there just to make your watch look snazzy.