# F-ckin Leap Seconds, how do they work?

Brett Frankenberger rbf+nanog at panix.com
Wed Jul 4 17:44:40 UTC 2012

```On Tue, Jul 03, 2012 at 04:54:24PM -0400, valdis.kletnieks at vt.edu wrote:
> 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.

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.

Without leap seconds, the sun stops being overhead at noon.  Without
leap years, the equinozes and solstices start drifting to different
days.

-- Brett

```