Why there is a difference between the days taken by a moon to complete the whole revolution and the again come to back to form a full moon?

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Solution

It takes 27 days, 7 hours, and 43 minutes for our Moon to complete one full orbit around Earth. This is called the sidereal month, and is measured by our Moon's position relative to distant “fixed” stars. However, it takes our Moon about 29.5 days to complete one cycle of phases (from new Moon to new Moon). This is called the synodic month. The difference between the sidereal and synodic months occurs because as our Moon moves around Earth, the Earth also moves around our Sun. Our Moon must travel a little farther in its path to make up for the added distance and complete the phase cycle.

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