Leap seconds are irregularly spaced because Earth’s rotation changes irregularly. Any events or processes that cause a significant redistribution of mass will change the length of the day by a small but measurable amount. These processes change the Earth’s moment of inertia, which affects the rate of rotation due to conservation of angular momentum. Tidal friction with the Moon is the most influential of these processes, lengthening each solar day by about +2.3 milliseconds per century. Another notable influence is glacial isostatic adjustment (redistribution of mass due to glacial activity), which currently shortens the solar day by -0.7 milliseconds per century. Other factors include the movement of the Earth’s crust and changes in mantle convection. It is estimated that the indian Ocean earthquake of 2004 shortened the day by -2.68 microseconds. Using historical records from the last 27 centuries along with new data, it is shown that, on average, the solar day is currently being lengthened by about +1.7 milliseconds per century.