The main reason for the slowing down of the Earth 's rotation is tidal friction , which alone would lengthen the day by 2.3 ms/century . Other contributing factors are the movement of the Earth 's crust relative to its core , changes in mantle convection , and any other events or processes that cause a significant redistribution of mass . These processes change the Earth 's moment of inertia , affecting the rate of rotation due to conservation of angular momentum . Some of these redistributions increase Earth 's rotational speed , shorten the solar day and oppose tidal friction . For example , glacial rebound shortens the solar day by 0.6 ms/century and the 2004 Indian Ocean earthquake is thought to have shortened it by 2.68 microseconds . It is evident from the figure that the Earth 's rotation has slowed at a decreasing rate since the initiation of the current system in 1971 , and the rate of leap second insertions has therefore been decreasing
2004 Indian Ocean earthquake
2004 Indian Ocean earthquake and tsunami
Leap second