From
The Gospel According To Newton :
An object in motion tends to stay in motion unless acted upon by an outside force. Now, apply this to the
Earth. The
Earth is a big spinning
rock. But that
rock has
oceans of air and water on it that are somewhat
independent of the rock; as
tides show, other
planetary (outside)
forces act on them. And, since the
wind blows on
mountain ranges and the
tides cycle
opposite of the
direction the
Earth is spinning, they are
gradually, bit by bit,
slowing the
Earth's
rotation down.
Each century, each solar day gets longer by 1.2 milliseconds.
Seems like small change, right? It used to be. But now that we run off of atomic clocks that are accurate to 10 femtoseconds, it's kind of important to know that the Earth's way of keeping time (rotating on its axis) is much less accurate then mankind's best technology. But, since the day is defined off of the Earth's rotation, and the second is not defined off of the day, the 'leap second' becomes a necessity.
The second was standardized in 1967 as
the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom
which means that the
second is now
fully independent from the
day. But we still expect
60 seconds a minute,
60 minutes an hour, and
24 hours a day, so to
correct for the
Earth slowing down and making each
day a
millisecond longer, a
leap second is added on to some years. At the stroke of
midnight on the
New Year, the atomic clocks show 11
:59:
58, 11:
59:59,
11:59:60,
12:00:
00. And those who aren't
aware of this leap but depend on
to-the-second accuracy in their
operations always get
screwed up by this change.