[btilly]

Exactly. The right hand rule.

And it isn't coincidence that stuff tends to spin the same way. Because when you don't then tidal forces try to pull you back into alignment. Of course, being basically frictionless tops, the main effect is that the rotation precesses. (The Earth's rotation takes about 26,000 years to precess.) However there is a tiny tidal friction component that slowly tends to bring things into alignment, and which is trying to lock all spins to each other.

And the operative word here is "slowly". If the Moon had been thrown off, spinning the opposite way from the Earth, it would have remerged. Instead the Earth's rotation is being transferred into throwing the Moon into a higher orbit at about 4 cm/year. (For comparison, this is a bit under twice the rate that the North American Plate is moving.)

The main consequence here on Earth is that the UTC system that we all use is going to have to add leap seconds more and more often over time, and in a few thousand years either we'll need to reconsider either the length of the second, or our tying the measurement of time to astronomy. (My vote would be to remove the astronomical definition. Humans seem to be OK living with time zones that do not match the sky. Having to move those time zones every few centuries does not seem like that big a deal. There won't be an observable issue in a normal human lifespan from this until after longer than human civilization has existed so far.

Astronomers are in charge of UTC right now, and they would hate the change. But when you consider how much it would cost astronomy to cope compared with the real costs of software bugs that tend show up when they insert leap seconds in UTC, right now the tail is very much wagging the dog.

[glurgh]

> reconsider either the length of the second

This one is done - the definition of the second is not based on astronomical observations.

"Since 1967, the second has been defined to be:

the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom"

The accuracy of this kind of measurement is so high it's also used (along with the speed of light) as the basis for defining the meter. Earth-time is a complicated and messy business but the base unit, the second, is pretty much nailed down and independent.

[btilly]

I was stating logical possibilities, not feasible ones.

You are right in how we define the second. It was chosen so that on average the number of seconds from midnight to midnight is 606024 = 86,400. At this point a ton of stuff depends on it - the only change that I think likely is to something that we can measure even more precisely than the current standard.

But we have set up UTC such that the first second of every year starts close to midnight for the Royal Observatory at Greenwich, London. This sometimes requires having minutes with 59 or 61 seconds (usually 61).

These weird minutes have real costs. For instance the June 30, 2012 leap second crashed Linux machines around the world, taking down quite a few websites. And the costs are only projected to increase as we have more programs that make assumptions about UTC, and the slowing day requires more leap seconds to be added.

Therefore logically we have 3 choices. Change the meaning of a second (not feasible), continue putting up with leap seconds (costly, and serves no purpose unless you're an astronomer), or give up the UTC peg to an astronomical fact. Unfortunately astronomers control UTC right now, don't want to give it up, and don't care how much their leap seconds cost everyone else. But this can't last forever.

Given the way that things work, I don't think that it will change until there is some disaster that is too big to ignore. For instance a leap second bug causes some critical control software to fail, leading to a industrial accident, which brings the attention of politicians and the general public to this ridiculous situations. (Or crashes critical aviation software. Or something else on that scale.)

If the current situation maintains, it is just a question of time until something on that scale happens. But until that happens, I don't think that astronomers will understand the costs that they are imposing on the rest of the world.

My thinking is that since it is obvious that we'll lose the astronomical peg eventually, we should lose it now. Before someone dies.

[glurgh]

This is a genuine question - do you really believe the (to me, seemingly obvious and much greater and potentially more dangerous) costs of switching away from astronomical time for measuring something inherently astronomical like the length of a year are smaller than the costs of removing the occasional time-handling bug in software?

Even in your example, Linux machines didn't actually crash - some processes spinlocked. And it wasn't even all Linux machines, if you were running a mildly old kernel (say, the one that came with debian etch and derivatives) nothing happened whatsoever.

Beyond that, there is no shortage of time standards that are both based on SI seconds and free of leap seconds - Terrestrial Time, International Atomic Time, GPS Time come to mind -

http://en.wikipedia.org/wiki/Terrestrial_Time

http://en.wikipedia.org/wiki/GPS_time#Timekeeping

http://en.wikipedia.org/wiki/International_Atomic_Time

Google's solution is 'leap smear' and they, similarly, did not run into the problem

http://googleblog.blogspot.com/2011/09/time-technology-and-l...

[lmm]

Equally genuine counter: what costs do you see associated with redefining a year as e.g. a fixed number of seconds? To me none are obvious, surely they'd be small compared to what we've already lost from these software bugs.

Leap smear is a good solution, as is TA1; sadly the posix/unix standards do not permit it (which makes it difficult to use on e.g. government projects).

[glurgh]

Well, for one thing a non-constant second seems like it would break just about all of physics in deeply unpleasant ways.

I think 'btilly is right - it's an unpleasantly difficult problem.

I suppose my point is 'it just happens to be unpleasantly hard problem' - given that a great number of specialists have put serious time in thinking about it trying to solve it, I have a very difficult time imagining that the solution (or satisfactory answer) is as simple as 'just blame astronomers'.

Having _two_ or more slightly different units that measure the base unit of time (which would be the case if we defined seconds in terms of (siderial? some other?)) year seems like a far greater potential source of cock-ups than just having a well-defined unit that might not quite, but more often than not, matches astronomical detail.

[lmm]

>I suppose my point is 'it just happens to be unpleasantly hard problem' - given that a great number of specialists have put serious time in thinking about it trying to solve it, I have a very difficult time imagining that the solution (or satisfactory answer) is as simple as 'just blame astronomers'.

As far as I can tell the proposal before the ITU (replace leap seconds with leap hours, with a view to abolishing leap hours once any incompatibilities with national laws have been resolved) solves all the problems quite nicely. I'm not entirely clear why they voted to defer a decision until 2015, but I'm not aware of any serious objections to the proposal.

[btilly]

This is a genuine question - do you really believe the (to me, seemingly obvious and much greater and potentially more dangerous) costs of switching away from astronomical time for measuring something inherently astronomical like the length of a year are smaller than the costs of removing the occasional time-handling bug in software?

Absolutely.

As http://queue.acm.org/detail.cfm?id=1967009 notes, fear of encountering leap second bugs ALREADY causes many factories to schedule downtime around leap seconds because of how serious the potential consequences of not doing so could be. It is just a question of time until someone who had such a bug doesn't realize it, doesn't schedule downtime, and suffers the consequences.

And what's the consequence of eliminating leap seconds? In the next few decades the Earth and our clocks will drift out of alignment by about 1% of the amount that they are already forced to be out of alignment for most of us by the fact that time zones are an hour wide. Which very few people will notice.

Even in your example, Linux machines didn't actually crash - some processes spinlocked. And it wasn't even all Linux machines, if you were running a mildly old kernel (say, the one that came with debian etch and derivatives) nothing happened whatsoever.

If the bug had been introduced during a longer period without leap seconds - not long ago there was one that lasted 7 years - then its impact would have been more widely felt. And it is true that it wasn't technically a crash, but many websites did suffer outages because internal services stopped responding. In common usage that was a crash.

Beyond that, there is no shortage of time standards that are both based on SI seconds and free of leap seconds - Terrestrial Time, International Atomic Time, GPS Time come to mind...

UTC is the standard for time in C, all languages derived from C (eg Java), and all languages written in C (eg Perl, Python, PHP, etc). It is also the standard in all Unix operating systems (including OS X), POSIX, Linux in all variations including Android, and has even been adopted by Microsoft. It is also the standard in widely used protocols written on top of that, like HTTP, and inside of any software built on top of them, such as web browsers.

Yes, there are other definitions of time that you can use. But UTC as published by NIST and distributed by the NTP utility is "the time" as far as the computer world cares. If you use anything else, people will ask - repeatedly - why you have the wrong time.