Leaping seconds

The Earth's spin is slowing down -- in a complex, non-linear fashion. The changes cannot be estimated more than six months ahead, just observed.

Originally, the second was defined as 1/86400 of a mean solar day, as determined by astronomical observation of the rotation of the Earth around its axis and its orbit around the Sun.

However, the solar day has gradually become 1.7 ms longer every century, due mainly to the friction associated with tides. [via Wikipedia and /.]