 Due to their tidal interaction , the Moon recedes from Earth at the rate of approximately 38 mm / yr . Over millions of years , these tiny modifications — and the lengthening of Earth 's day by about 23 µs / yr — add up to significant changes . During the Devonian period , for example , ( approximately 410 mya ) there were 400 days in a year , with each day lasting 21 @.@ 8 hours . 
