section_path: Variations in length
section_name: Variations in length
paragraph_text: Variations in length
  
  Mainly due to tidal deceleration – the Moon's gravitational pull slowing down the Earth's rotation – the Earth's rotational period is slowing. Because of the way the second is defined, the mean length of a solar day is now about 86,400.002 seconds, and is increasing by about 2 milliseconds per century.
  
  Since the rotation rate of the Earth is slowing, the length of a SI second fell out of sync with a second derived from the rotational period. This arose the need for leap seconds, which insert extra seconds into Coordinated Universal Time (UTC). Although typically 86,400 SI seconds in duration, a civil day can be either 86,401 or 86,399 SI seconds long on such a day. Other than the two-millisecond variation from tidal deceleration, other factors minutely affect the day's length, which creates an irregularity in the placement of leap seconds. Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service (IERS), which measures the Earth's rotation and determines whether a leap second is necessary.