4 ms·

Good ol' cross-product and angular momentum!

by cefarix 13y ago

btilly 13y ago
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 13y ago
  > 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 13y ago
    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 13y ago
      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/Terrestrial_Time http://en.wikipedia.org/wiki/GPS_time#Timekeeping http://en.wikipedia.org/wiki/GPS_time#Timekeeping http://en.wikipedia.org/wiki/International_Atomic_Time 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-leaping-seconds.html http://googleblog.blogspot.com/2011/09/time-technology-and-l...

Good ol' cross-product and angular momentum!

by cefarix 13y ago

Good ol' cross-product and angular momentum!

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 13y ago
> 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 13y ago
  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 13y ago
    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/Terrestrial_Time http://en.wikipedia.org/wiki/GPS_time#Timekeeping http://en.wikipedia.org/wiki/GPS_time#Timekeeping http://en.wikipedia.org/wiki/International_Atomic_Time 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-leaping-seconds.html http://googleblog.blogspot.com/2011/09/time-technology-and-l...