It would seem that defining the second would be an easy task. It is 1 60th
of a minute, and therefore 1 86,400th of a day. In fact, until the 20th century
this was the official definition of the minute.
It had not always been the definition. After all, for the first few millennia
of timekeeping no exceedingly accurate device existed, and there was no need
for an exact definition of the second. The original hours did not even have
strict definitions.
Unfortunately, the Earth is not an exceedingly accurate timekeeper either.
The rotation of the Earth is constantly slowing. While only fractions of
fractions of a second, it is slowing down and providing longer and longer days.
Trying to follow the length of the day by the position of the Sun is also
difficult, as there are always variations between one day and the next.
In 1967 it was decided that a new exact definition which would never change
would be given for the second. The 13th General Conference of Weights and
Measures determined that the new definition of the second would be
9,192,631,770 vibrations of the caesium atom (the same measurement used to determine
time with the atomic clock). This is now the official definition of the second,
and our minutes, hours and days are also based upon this definition.
Leap Seconds
Because the second is no longer defined by the rotation of the Earth, it is
required to occasionally make adjustments to the clock to keep us in line with
astronomical time. Like leap years, we also have leap seconds, where clocks are
put back by a single second. Being so minor, these leap seconds generally go
unnoticed, although they have been happening since 1972.