What changes during the lunar cycle?

The lunar phases gradually change over the period of a synodic month (about 29.53 days), as the orbital positions of the Moon around Earth and of Earth around the Sun shift. The Moon's rotation is tidally locked by Earth's gravity; therefore, most of the same lunar side always faces Earth.

Explanation:

The lunar phases gradually change over the period of a synodic month, as the orbital positions of the Moon around Earth and of Earth around the Sun shift. The Moon's rotation is locked by Earth's gravity; therefore, most of the same lunar side always faces Earth. (Cited from Wikipedia)

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