What causes phases again?
The difference in the moon's position in relation to the sun causes the phases.
When one observes the phases of the moon he is simply observing the moon's day and night, a natural shadow from the sun illuminating half of the spherical moon at any one time.
The lunar phases vary cyclically according to the changing geometry of the Moon and Sun, which are constantly wobbling up and down and exchange altitudes as they rotate around the North Pole.
When the moon and sun are at the same altitude one half of the lunar surface is illuminated and pointing towards the sun, This is called the First Quarter Moon. When the observer looks up he will see a shadow cutting the moon in half. The boundary between the illuminated and unilluminated hemispheres is called the terminator.
When the moon is below the sun's altitude the moon is dark and a New Moon occurs.
When the moon is above the altitude of the sun the moon is fully lit and a Full Moon occurs.
The time between two full moons, or between successive occurrences of the same phase, is about 29.53 days (29 days, 12 hours, 44 minutes) on average. This denotes the cycle of alternating altitudes.