What is a second? | Hacker News Reader

What is a second?

johndcook.com

75 pointszdw33 comments

A funny consequence of this is that there are people alive today that do not know (and never will know) their exact age in seconds[1].

This is true even if we assume the time on the birth certificate was a time precise down to the second. It is because what was considered the length of a second during part of their life varied significantly compared to what we (usually) consider a second now.

[1] Second as defined in the article as 9192631770/s being the the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom

I guess the Python datetime module needs some more functions then.

You really need one extra field for that though: uncertainty.

Even then, you run into issues with gravitational time dilation (though at scales much smaller than whole seconds). For instance, the TAI timescale was quietly changed at one point to follow a new reference altitude, causing the 'calendar second' to run slightly differently with respect to any stationary observer. The only way to really know your age in your own proper time is to have an atomic clock following you everywhere since birth.

Or perhaps surgically appended into your palm or something...

That would be the ultrahigh-precision version of Logan's Run.

Technically your hands could (or probably) have a different age than e.g. your brains.

They would for sure because they move independently from brain. They will also tend to be slightly lower most of the time, so the gravity they experience is slightly larger affecting time passage. The relativistic effects of hand motion on age would be small of course, but we are talking about extreme precision here.

Is the moment of birth defined down to the second? The process of giving birth seems more like a process (to this lay person who failed high school biology) rather than a strict binary not-yet-born/born.

https://en.wikipedia.org/wiki/Leap_second

It seems like there's not much agreement on what to do about -- or whether to even use -- leap seconds.

Sounds like a fairly complex problem to solve, given such a relatively miniscule amount of time -- possibly even tougher than solving leap year and/or time zones/DST.

A thought I've had is we should ignore leap seconds until the difference between UTC and timezone reference times (e.g. mean midnight at the Greenwich meridian) reaches some significant threshold, e.g. 5 minutes. At this point, all timezones worldwide should perform a coordinated shift by the threshold amount - so Europe/London moves from UTC+0000 to UTC+0005, Eastern Standard Time moves from UTC-0500 to UTC-0455, etc... and stay there until the difference reaches the threshold again (one way or another). We should get at least a couple of decades of forewarning if such a shift is likely to be needed, which should be enough to plan for and test the first migration.

Whether this co-ordination involves everyone switching at the exact same instant, or each timezone switching at e.g. 01:30 in that timezone, is left as an exercise for the reader.

Also... it pushes the first change far enough in the future that the people designing the rules likely won't have to worry about having to implement them. Which might make it easier to agree on them ;-)

This, although we don't even need to agree to do anything (just agree to so nothing before some threshold).

Setting timezones is inherently political, so politicians are free to decide what to do when (as long as we can agree that shifting timezones a second at a time is silly).

When the time comes they may even decide to do nothing at all, or delay things until 15 min have passed, or who cares…

> We should get at least a couple of decades of forewarning if such a shift is likely to be needed, which should be enough to plan for and test the first migration.

A migration that only happens every couple of decades is guaranteed to be one we never get good at, and flub every time.

> A migration that only happens every couple of decades is guaranteed to be one we never get good at, and flub every time.

Related: we have leap years regularly, and yet people flub February 29 regardless of the predictable occurrences.

I hadn't thought about that, but I'd actually like to see if it's better to do the opposite. Insert half a leap second twice as frequently as we currently do with a leap second. If that's successful, switch to inserting a quarter leap second and so on, maybe until we get to a state where we adjust a tiny fraction of a leap second on a daily basis. The point is that even if you have a system screw up time adjustment 1 day, it's such a small time error that shouldn't affect anything, and people can immediately work on fixing it.

And actually, what you explained about waiting until some significant threshold like 5 minutes is already something we do today in a way. It's just that we wait years and our threshold is 1 second before we actually make a change.

I don't think the threshhold is that significant. The main difference I'd like to see is that we apply the change to timezones, not to the underlying "count number of seconds" clock. But I think timezones aren't able to be specified to sub-minute precision, so 5 minutes is just as good as 1 minute.

> Incidentally, “cesium” is the American English spelling of the name of atomic element 55, and “caesium” is the IUPAC spelling.

I'm so glad we settled on common notations (drived from Brahmi script) to represent numbers. On one end, we humans chase precision with math. On the other end we humans can't even agree on spellings in the same language spoken in different continents. I wonder if spelling differences has resulted in any critical failures such as the metrics conversion error in NASA Mars Climate Orbiter in 1999 https://en.m.wikipedia.org/wiki/Mars_Climate_Orbiter

What we want:

1. A second has fixed length. 2. At 12 noon every day the sun is at its zenith. 3a. A day has 86400 seconds. 3b. Time only goes forward.

Pick any three.

not 1; 2, 3a, 3b: You get UT1 [1], which basically gives you the angle of the earth wrt the sun. Time proceeds continuously with 86400 seconds a day, but they're of varying length (depending on the earth rotation's speed) to keep up with where the sun is. This is measured by the International Earth Rotation Service [2].

1, not 2, 3a, 3b: You get TAI [3]. Time proceeds continuously in SI seconds with 86400 seconds a day, but 12 noon slowly goes out of sync with the earth rotation.

1, 2, not 3a, 3b: You get UTC. It's using SI seconds, tries to keep in sync with the earth's rotation, but therefore must occasionally include leap seconds, so there will sometimes be 86401 seconds in a day (the watch will display 23:59:59 for a second and then 23:59:60 for a second).

1, 2, 3a, not 3b: You get UNIX time [4]. It's using SI seconds, tries to keep in sync with the earth's rotation, but when a leap second occurs, it will "jump back" and repeat a second (the watch will display 23:59:59 for two seconds).

[1] https://en.wikipedia.org/wiki/Universal_Time

[2] https://en.wikipedia.org/wiki/International_Earth_Rotation_a...

[3] https://en.wikipedia.org/wiki/International_Atomic_Time

[4] https://en.wikipedia.org/wiki/Unix_time

At the risk of being "that guy", a day is not the time it takes the earth to rotate on its axis.

Yes, the earth has rotated on its axis and a little bit more in a solar day (361°, 24h0m). You can count sidereal days instead (360°, 23h56m) if you want to stop after exactly one rotation.

This article is "being that guy" and I love it. I assume you are referring to solar day?

Thank you, I was going to point that out myself. To let you not feel alone, I'll be “that guy” with another one:

The article talks about “when the dinosaurs roamed the earth”. Even ignoring for a moment that they're still roaming just fine today (birds are dinosaurs), this pretends like there was a singular moment when dinosaurs were “roaming”. In reality, of course, dinosaurs have existed for hundreds of millions of years; way longer than twice the time since the K-Pg extinction event (which, again, didn't fully cause all dinosaurs to go extinct).

In English, the word "when" can validly refer to an arbitrarily long period of time, not just a point in time.

To be more pedantic, doesn't that depend on your choice of coordinate system?

> Here “tropical year” means the time it took earth to orbit the sun from the perspective of the “fixed stars,”

Isn't tropical year relative to sun, i.e. equinox to equinox? And sidereal year being relative to fixed stars?

I was looking for a human scale unit of time that was proportional or a harmonic of some other universal physical constant, this reconciliation of the cesium clock second with historical definitions is probably why I couldn't find one.

turns out it's a measure of arbitrary precision.

So where does that leave all those incredibly expensive watches that boast accuracy?

Question asked, question answered...

What is a second? (johndcook.com)

An Unreasonable Amount of Time (allenpike.com)

a 4 hour day/night cycle would be wild

Can’t imagine humans could tolerate life in that rapid cycle.

Fortunately there were much more pressing reasons why humans could not tolerate life on Earth immediately after the formation of the moon. :)

yeah that’s a curious thought experiment, adapting to it from 24 hour days suddenly would be one thing but what if it had always been 4 hours, maybe we would do things slower rather than faster and there would be eating days and chore days, if we were calibrated to sleep for 1-2 hours instead of 8… or would we move faster? and there may not even be “hours” at all as we know them, maybe we would divide things up into 15 minute chunks, sounds like a short story waiting to be written, have a guy from earth visit a planet with humanoids that live on 4 earth hour day cycles and he is observing their living patterns while dealing with the pain of adapting from his normal

Not to be confused with a second of arc, which is 1/3600th of a degree. A third is 1/60 of that.

Reply on news.ycombinator.com