The phases of the Moon are caused by the constantly changing positions of the Sun and Moon relative to the Earth, as shown on the diagram. The inner circle shows the changing position of the Moon, the outer circle illustrating how the Moon appears to us at each point in its orbit.
New Moon occurs when the Moon is positioned between the Earth and Sun (1), the Moon’s illuminated hemisphere facing away from the Earth. As the Moon moves along its orbit, more and more of its illuminated hemisphere becomes visible to us, the lunar phase gradually expanding (waxing) through crescent (2), first quarter (3) and gibbous (4) to Full Moon (5), following which the sequence is reversed (waning) until the following New Moon.
It is important to note that, although the Moon takes 27.32 days to complete one orbit, the interval between successive New Moons is 29.53 days. This is because the Earth and Moon are travelling together around the Sun, producing a delay between the completion of one full lunar orbit and the realignment of the Earth, Moon and Sun. In other words, the Moon has to travel a little bit further than 360° (relative to the Earth) in its orbit in order for it to ‘catch up’ and become realigned with the Earth and Sun and for New Moon to occur again.