"tidal friction"
by cm

Tidal Braking
Creative Commons Attribution Share AlikeTidal Braking - Credit: AndrewBuck, Wikimedia
The Moon’s gravitational pull deforms the shape of the oceans and even the solid mass of the Earth.   The resulting tidal bulge, when offset from the Earth-Moon axis, creates torque between the Earth and the Moon.   This converts the Earth’s rotational energy into heat energy, effectively braking the Earth’s rotation.   Over the next one hundred million years, our 24hr day will increase in length by about one hour.