F-ckin Leap Seconds, how do they work?

Thu Jul 5 02:01:50 UTC 2012

On Wed, Jul 04, 2012 at 05:02:02PM -0400, valdis.kletnieks at vt.edu wrote:
> On Wed, 04 Jul 2012 12:44:40 -0500, Brett Frankenberger said:
> 
> > Leap Seconds and Leap Years are completely unrelated and solve two
> > completely different problems.
> >
> > Leap Seconds exist to adjust time to match the Earth's actual rotation.
> > They exist because the solar day is not exactly 24 hours.
> >
> > Leap Years exist to adjust time to match the Earth's actual revolution
> > around the Sun.  They exist because the that time period isn't exactly
> > 365 days.
> 
> Actually, it's the same exact problem - an astronomical value isn't
> exactly conformant to the civil value, and thus adjustments are needed.

No.  Leap Years arise because the solar year is not an integral
multiple of the solar day. 

Yes, you can argue that leap years exist because the Earth doesn't
revolve around the sun in 86400*365 seconds, but that missed the
underlying point that since well before civil time differed from solar
time, people have defined a year in terms of days, preferring not to
have years starting a midnight, then dawn, then noon, then dusk, and so
on.  Leap years have existed since well before civil time and solar
time were any different.

> And you missed the bigger point - that leap seconds aren't needed because the
> earth is slowing any more than leap days are needed because the year is getting
> longer.  If an actual siderial day was a fixed unchanging 86400.005 seconds
> long, you'd still need a leap second every 200 days.  *SLOWING* would be
> indicated by the "every 200 days" changing to "every 175" or "every 150".

I assume you meant "solar" instead of "[sidereal]" -- the sidereal day
hasn't been 86400.anything seconds ever.  And if the mean solar day
were unchanging, then it would be 86400 civil seconds today, just like
it was (by definition) in 1900.  The civil second was initially
defined as 1/86400 of the mean solar day in 1900 (then later redefined
based on radiation from the cesium atom, but the redefinition didn't
change the length of the second by enough to matter for the purposes of
this discission).  The only reason the mean solar day today isn't 86400
is because the Earth's rotation has slowed since 1900 and we've elected
to not redefine the length of a second.

Yes, technically, you're right that if the Earth's rotation rate were
constant and were such that the mean solar day were 86400.005 seconds
long, we'd still need leap sections.  But that's a highly unlikely
counterfactual hypothetical, because, again, if the Earth weren't
slowing, then 1/86400-of-mean-solar-day defintion of the second would
still hold.  There's virtually no chance that on a hypothetical Earth
that wasn't slowing, that population would have decided that the
second should be 1/86400.005 of a solar day.

     -- Brett