Question

what is the reason behind that the earth is now revolce slower than earlier

Answer 1

Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds.

Answer 2

Earth rotates, the Moon's gravity causes the oceans to seem to rise and fall. ... There is a little bit of friction between the tides and the turning Earth, causing the rotation to slow down just a little. As Earth slows, it lets the Moon creep away.