In an effort to match clocks to the Earth's slowing spin on its axis, the world's official timekeepers have added a "leap second" to the last day of 2008. The Earth's rotation is affected by a host of slightly fluctuating variables, including the movement of the tides, changing polar ice caps, space dust, solar wind, and magnetic storms. Since the exact speed of the Earth's rotation cannot be charted in advance, the periodic insertion of a leap second into the atomic time scale is needed to keep the system aligned with mean solar time. This will be the 24th leap second added to coordinated universal time since 1972, when the practice was first instituted.