The secular change in the planet's rotation is a classical topic in geophysics. It goes back some 300 years to when Sir Edmond Halley first hypothesized that the moon was accelerating in its orbit. Most of Halley's lunar acceleration was only apparent. It was actually the earth's rotation slowing down, making the moon appear to accelerate. The moon does accelerate (strictly, it decelerates), but the larger effect is the earth's rotational braking. This braking is caused by tidal friction. Throughout the earth's history tidal braking has played, and it will continue to play, a dominant role in the rotation. Currently the secular change in the rotation rate increases the length of day by some 2.3 milliseconds per day per century.