Scientists reveal 2016 will get an extra ‘leap second’

Already a leap year, 2016 will drag on a bit longer.
International timekeepers are adding a leap second at year’s end.
Because Earth is slowing down very slightly, occasionally timekeepers insert another second or two to match Earth’s rotation and the precise atomic clocks.

Historically, time was based on the mean rotation of the Earth relative to celestial bodies and the second was defined in this reference frame.
However, the invention of atomic clocks defined a much more precise ‘atomic’ timescale and a second that is independent of Earth’s rotation.