Answer:
The main cause of the moon phases is the changing position of the moon in relation to the sun and the earth.
Step-by-step explanation:
The moon phases are the perception of the part of the moon that is visible, when observed from the earth. When it is completely visible it is called full moon, when it cannot be seen it is called new moon and the phases between one and the other are called crescent and waning quarters.
The moon can be visible when it receives illumination from the sun but, due to its changes in position with respect to the earth, this illumination can be complete, partial or absent, which defines each of the phases.
When the moon completes its four phases it is called the lunar cycle, and it has an estimated duration of 29.5 days.