F-ckin Leap Seconds, how do they work?

valdis.kletnieks at vt.edu valdis.kletnieks at vt.edu
Tue Jul 3 20:54:24 UTC 2012

On Tue, 03 Jul 2012 21:49:40, Peter Lothberg said:

> Leapseconds can be both positive and negative, but up to now, the
> earth has only slowed down, so we have added seconds.

That's what many people believe, but it's not exactly right.  Leap seconds
are added for the exact same reason leap days are - the earth's rotation
isn't a clean multiple of the year.  We know we need to stick in an entire
leap day every 4 years or so, then add the 400 hack to get it closer. At
that point, it's *really* close, to the point where just shimming in a second
every once in a while is enough to get it back in sync.

The earth's slowdown (or speedup) is measured by *how often* we
need to add leap seconds.  If we needed to add one every 3 years, but
the frequency rises to once every 2.5 years, *that* indicates slowing.
In other words,  the slowdown or speedup is the first derivative of
the rate that UT and TAI diverge - if the earth rotated at constant
speed, the derivative would be zero, and we'd insert leap seconds on
a nice predictable schedule.
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