The rotation of the earth on its axis is periodic motion but not oscillatory. why?
Periodic motion is the motion in which the moving body returns to the same condition after regular (fixed) intervals of time.The motion of the Earth round the sun is periodic because it always completes one revolution in one year.
The motion of the earth rotating on its axis once in 24 hours is a kind of periodic motion because the time duration of 24 hours is fixed.The rotation of the earth on its axis is periodic motion .
Oscillatory motion is that motion in which body repeatedly moves to-and-fro about a fixed point.Earth's motion is not to-and-fro about a fixed point.