Velocity blindness

[gfish]

When I was researching clock design extensively last year, I had an odd linguistic revelation. All my life I'd been saying a clock was "fast" or "slow" to indicate it was ahead or behind the correct time. This purely meant a fixed offset -- a clock was 5 minutes fast, and would still be 5 minutes fast next week. In fact, if a clock really WAS fast, getting further and further ahead I'd have to explicitly specify that. It was a sloppy use of "fast", but, eh, natural language is full of that. I never really thought about it. It wasn't until getting into the history of clocks that I realized this hadn't always been an idiosyncratic phrasing. It used to be quite literal. Clocks used to suck, even very expensive ones. If your clock was fast, it would be noticeably gaining time day after day. We live in an era of incredibly cheap, incredibly accurate clocks. Even the cheapest quartz wristwatch might only gain a few seconds over an entire year, a level accuracy that used to only be found in the best observatory regulators. So while the immediate observation is the same (a clock is 5 minutes ahead), the underlying condition is completely different. It's like seeing a plane taking off over a continental plate boundary and thinking they both have the same velocity since they're the same distance away from you. We take it so granted that clocks are (for >99.99% of human activity) perfectly accurate that we have taken the indefinite integral of a common figure of speech without even realizing it. That's pretty cool. I can't think of any other examples of that happening.

Comments

[anansi133]

This reminds me of the insanely accurate values of pi that people like to play with- where you need a circle the size of the solar system to talk about how far out of round the number would get you.

So now I'm suddenly curious how accurate is close enough so that no one notices. In sailing terms I guess it's being able to get within eyeshot of a small island, but in urban terms it's probably making a bus rendezvous.

When I got my first GPS my favorite part was the super accurate clock. didn't need it, but it was fun anyway.

[neuro42.livejournal.com]

I think not. What you're missing is the implied presumption that (a) the clock was once set and (b) at that time it was correct.

Therefore, in order for it to *now* be five minutes ahead, it *must* be running at fast by (5m/dT) seconds per second, and the reasonable expectation is that it will continue to do so.

Noboody (that I know of) thinks a clock that's 5m ahead will stay 5m ahead forever. It might take more than next week, but one can only expect it to keep gaining.

[peteralway.livejournal.com]

It actually took me some time to accept that when people said a watch was "fast" that it was really shorthand for the watch being ahead at the moment. Just seemed wrong. Especially because my motivation for setting my cheap timex 5 minutes ahead was because it was in fact slow, and I wanted a safety margin as it fell behind.

[solarbird]

My battery backup bedroom clock - digital! - actually is fast in the old sense. It's about four minutes fast across six months, which is still not horrible, but it does mean every DST adjustment involves individual minute changes. I still use it because I love the way I push a button and it unfolds from a slab into this clock-and-calculator thing, but honestly, by modern standards? It's a pretty poor clock.

[sistawendy.livejournal.com]

Yes! Yes! Somebody else who noticed the inaccuracy! Fish, you are my people.

And yeah, I knew that about the old clocks sucking. Shoot, among spring-wound clocks, even recent ones suck if they're cheap enough or the spring is shot.

[randomdreams.livejournal.com]

I'd internalized this without thinking of it: I say 'ahead' when the clock's set 5 minutes fast but is otherwise reliable, because I've spent a lot of time messing about inside vintage mechanical wristwatches. The mainspring attaches to a central pivot, which has an arm on it, allowing it to be varied. That changes the spring tension, which varies the watch speed. The arm end has a pointer on an inscribed arc, that's labeled "fast" and "slow" on the two ends.
Thing is: until you pointed this out, I had never consciously thought about my usage of ahead/behind vs fast/slow, so I must have been interpolating what other people were saying, unconsciously.

[zzyzx-xyzzy.livejournal.com]

The clocks on desktop PCs are surprisingly drifty on short timescales. I see drift rates on the order of 0.1 msec per second around here.