What causes moon phases? Why is there only one full moon a month?

Both Moon's day-night spin-period and its orbital period around Earth are the same 29.53 d and the same hemisphere faces Earth. The gradual day-night transit across this face is observed in phases.

During this period, Moon appears alternately in the (Earth-Moon-Sun) Sun side and (Moon-Earth-Sun) opposite side. While in the Sun-side, we see dark Moon. On the opposite side, the face we see is wholly sunlit, and it is full Moon. This period 29.53 d is the time between two successive Full Moons At New Moon and Full Moon, the Moon crosses the heliocentric (moving) longitudinal plane of . .