[LEAPSECS] ACM article

[Warner Losh]

On Apr 9, 2011, at 4:03 PM, Rob Seaman wrote:


> Hi Tom,

> 

>> Help me out here. That ACM generated time-stamp in your posting; which is it by your definition: time-of-day or interval timekeeping?

> 

> Universal time is time-of-day.  The current definition of UTC permits it to be used to recover an interval timescale.  Timestamps supply information about both time-of-day and an interval since an epoch.  More to the point the interval may be in evenly-tempered units or in jerked-about-by-the-moon units.  It may very well be of interest to different stakeholders whether the stamped event occurred at "noon" or rather whether it occurred so many thousands or millions of SI seconds before or after some other event.  The current definition of UTC preserves access to both.


It is actually the elapsed time in SI seconds since the fictions midnight, which differs from both solar midnight and mean solar midnight by some amount. This differs a little from the elapsed time in mean solar seconds as well.  Since it is expressed in SI seconds, which do not represent 1/86400th of the earth's rotation, they are a pure elapsed time that happens to be approximately the same as a time of day measurement.  The current definition of UTC doesn't answer the question of if it is an elapsed time or a time of day, since the difference between these two is small due to the addition of leap seconds.

So while UTC is steered to be a useful approximation of earth orientation, that does not make it a measure of earth orientation.

Warner