<P> Over millions of years, the Earth's rotation slowed significantly by tidal acceleration through gravitational interactions with the Moon . In this process, angular momentum is slowly transferred to the Moon at a rate proportional to r − 6 (\ displaystyle r ^ (- 6)), where r (\ displaystyle r) is the orbital radius of the Moon . This process gradually increased the length of day to its current value and resulted in the Moon being tidally locked with the Earth . </P> <P> This gradual rotational deceleration is empirically documented with estimates of day lengths obtained from observations of tidal rhythmites and stromatolites; a compilation of these measurements found the length of day to increase steadily from about 21 hours at 600Myr ago to the current 24 hour value . By counting the microscopic lamina that form at higher tides, tidal frequencies (and thus day lengths) can be estimated, much like counting tree rings, though these estimates can be increasingly unreliable at older ages . </P> <P> The current rate of tidal deceleration is anomalously high, implying the Earth's rotational velocity must have decreased more slowly in the past . Empirical data tentatively shows a sharp increase in rotational deceleration about 600Myr ago . Some models suggest that the Earth maintained a constant day length of 21 hours throughout much of the Precambrian . This day length corresponds to the semidiurnal resonant period of the thermally - driven atmospheric tide; at this day length, the decelerative lunar torque could have been canceled by an accelerative torque from the atmospheric tide, resulting in no net torque and a constant rotational period . This stabilizing effect could have been broken by a sudden change in global temperature . Recent computational simulations support this hypothesis and suggest the Marinoan or Sturtian glaciations broke this stable configuration about 600Myr ago, citing the resemblance of simulated results and existing paleorotational data . </P> <P> Additionally, some large - scale events, such as the 2004 Indian Ocean earthquake, have caused the length of a day to shorten by 3 microseconds by affecting the Earth's moment of inertia . Post-glacial rebound, ongoing since the last Ice age, is also changing the distribution of the Earth's mass thus affecting the moment of inertia of the Earth and, by the conservation of angular momentum, the Earth's rotation period . </P>

What is the period of rotation for earth