What factor is responsible for determining how often a full moon appears?

Dear Student,

The phases of moon appear due to rotation of moon around earth in its orbit. Moon completes one rotation in 27 days.
When moon rotates, there are relative positions of the moon, earth and sun. Due to these positions, earth comes in between the moon and sun and different phases are seen by observer.

Full moon occurs when the moon is in opposition to the sun (when it is on the opposite side of the Earth from the Sun)
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Regards,