[ntp:questions] Google and leap seconds

unruh unruh at wormhole.physics.ubc.ca
Wed Sep 21 19:54:35 UTC 2011

On 2011-09-21, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> On 9/21/2011 11:28 AM, Dave Hart wrote:
>> On Wed, Sep 21, 2011 at 15:06, Marco Marongiu<brontolinux at gmail.com>  wrote:
>>> Il 21/09/2011 16:51, Brian Utterback ha scritto:
>>>> Define "the right way".
>>> > From my original message, and D.Hart's reply, I'd say the right way
>>> would be that the system support either to have a second 60 after the
>>> second 59 (leap insertion) or that second 00 follows 58.
>>> But this would break POSIX...
>> Arguably this is what is done by ntpd now, within the important limit
>> that no platform on which ntpd runs supports a second numbered outside
>> [0-59].  So either the second numbered 59 is followed by another
>> second numbered 59, breaking the monotonically increasing requirement,
>> or similar with the second numbered 0, both of which amount to
>> stepping the clock back one second, or you choose to slew or "smear"
>> the one second adjustment over some longer period of time, possibly
>> stepping the visible clock frequency twice to do so, or using the more
>> sophisticated google cosine-shaped smear to limit the rate of change
>> of the leapsec-induced frequency adjustment.  The slewing approach has
>> the upside of no risky backward step, and the downside of up to 1s
>> offset vs. UTC during the slew.
>> Cheers,
>> Dave Hart
> It's unfortunate that the earth DOES NOT rotate exactly 360 degrees in 
> exactly 24.000000000000 hours. This bit of poor design causes all sorts 
> of problems.  Leap seconds are just one of the symptoms!

Oh, but it did. And then nature slowed down the rotation of the earth.
Of course one could have simply defined the second as 1/86400 of the
rotation period of the earth, but scientists instead decided to do
thinks like keep the frequency of Cesium atoms the same, and the speed
of light constant instead. 

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