It is additional one second that is added every 500 days to the International Atomic Clocks to compensate for the slowing down of the rotation of the earth largely due to tidal force of the moon.

Atomic Clocks: Atomic clocks work as the source of most accurate time and work on electron transition frequency.

• It is the source of International Atomic Time which forms the reference for the civilian timescale i.e. Universally Coordinated Time (UTC). Atomic Clocks in India are placed under CSIR.