Today's fun rabbit hole: there's a mechanical watch...

Today's fun rabbit hole: there's a mechanical watch feature called a "hack". Why is it called that?

The hack (aka a "hacking seconds watch") is a feature where the seconds hand of the watch stops advancing when you click over to the time-setting function. IOW, as you adjust the hour/minute, the seconds hand is frozen in place until you finish the time adjustment.

On 20th century wristwatches, this is implemented surprisingly simply: when you pull the crown out to adjust the time, a little spring called the hack moves and makes contact with the mechanical oscillator that makes the watch advance (it drives the pallet fork and escapement wheel, which produces the ticking sound)

The friction from the hack stops the balance oscillator dead in its tracks, and the watch stops ticking until the hack is released.

This mechanism became popular in WW2, because the US army put together specs for watches to be issued to soldiers, and one of their requirements was that all the watches must have a hacking functionality.

In war films, if you ever wondered what a bunch of soldiers standing in a circle shouting "hack!" at each other meant: they're synchronizing their watches. They use the hack to stop their watch's motion, line up their times, and release the hack all in sync.

For watch movements of the time, an ensemble of hacked watches could stay in sync to within about 1-2s per day, more than good enough for things like coordinating the time of an assault.

But... why's it called the hack? Why are these watches called hacking watches?

AFAICT, that goes back to the 18th century, and the dawn of precision timekeeping.

Ships on the open ocean navigated by the stars. Using a sextant, the navigator could establish with quite good precision the longitude of the ship, via some simple math... provided they knew the exact current time at Greenwich.

For a long time the challenge was that clocks and ships didn't go together. Clocks used big pendulums, and, well, ships roll and shudder and generally are not a good environment for swinging pendulums if you want them to be reliable. Plus, turns out gravity isn't constant! And the sub-1% variations in local gravity could lead to very significant error.

So, there was great interest in developing a timekeeping mechanism that didn't rely on gravity.

This was eventually solved, by the invention of the balance oscillator, which uses a coiled spring and a clever energy replenishment mechanism to provide a very precise oscillation movement that is independent of gravity. It depends only on the properties of the spring, and the discovery of the Elinvar alloy allowed for the manufacture of springs whose properties didn't vary with temperature - the last big obstacle to stability.

Amazingly, the balance oscillator was _such_ a good idea that it's still how mechanical watches are built to this day. With increasingly small refinements, but the fundamental movement of an 18th century ship's chronometer and a Rolex you could buy today use identical principles.

Anyway, problem solved, right? The balance oscillator allowed for the construction of chronometers that could keep time within a few seconds per year, which resulted in navigational error of just a few miles. By looking at the stars! Amazing.

However, there's still a problem. The ship's chronometer is very expensive, very delicate, and very picky about its environment. It wants to be isolated from the ship's movement, and doesn't like salty sea air (rust!).

So, the very expensive, finicky chronometer was kept below decks, in a dry room near the center of the ship, mounted in a gimbal so that it wasn't subject to forces from waves and the ship rolling, which would introduce error into the timekeeping.

But you may remember, you can't see the sky from a small room inside a ship.

And the navigational fixes from celestial navigation do care about the time down to the second, so you can't just get a fix in the sextant, then run downstairs and write down the time. What do?

Well, you issue the navigator a cheap, altogether more ordinary pocketwatch. Maybe it's good to within a dozen seconds per day. And cheap enough that it can be replaced if it rusts from the ocean air.

The navigator goes to the dry room, synchronizes their watch to the ship's chronometer, then goes above deck and takes their navigation measurements, using their pocketwatch as the time reference.

The navigator's watch, allegedly, was called the "hack watch". I think it's referring to the meaning of hack as a shortened form of "hackneyed", aka vulgar, commonplace, ordinary.

You have your very expensive, delicate ship's chronometer, which is very good indeed at timekeeping, and you use that as a reference to adjust a hack watch, which will keep the time well enough for the next 30min while the navigator needs it to.

So... I think that's where that comes from. The hack watch in the 18th century navy didn't have a way to stop the seconds hand, as far as I can tell, that only showed up in the early 20th century.

So, I _think_ this feature of mechanical watches is called the hack because the term carried over from celestial navigation in the navy, where the hack watch's primary purpose was to be synchronized to the ship chronometer for establishing position.

And so, at some point, the US military decided that the ability to synchronize soldiers' watches was important, and called that a hacking feature, presumably as a backreference to the navy term even though the application and purpose were pretty different.

Anyway, that's been my adventure down the paths of history to explain a 5mm springy metal rod that shows up in some modern wristwatches.