Hint: To complete one revolution of the earth around the sun it takes one year. The earth always takes this same amount of time to complete this cycle and repeats the same trend in the same period of time interval.
The revolution of the earth around the sun can be classified under a periodic motion. A periodic motion is a motion in which the motion repeats itself in a specific amount of time. The earth revolves around the sun every year, when it completes one revolution around the sun we experience a year. This motion repeats and hence it is a periodic motion.
This is similar to a pendulum working in vacuum, the bob always comes to the extreme position at regular intervals of time and repeats the same cycle. A periodic motion has three characteristics : velocity, period and amplitude. Period is the time taken for one cycle. Amplitude the maximum displacement from the mean position.
The rotation of the moon around the earth is also a periodic motion. Furthermore all planets revolve around the sun and all these motions are periodic motion, it repeats itself in a particular period of time. The earth requires 365 and a quarter days to complete one revolution. Similarly other planets which are closer to the sun require less time and planets which are farther require more time to complete one revolution.