Waxing gibbous Moon. Optical image of a waxing gibbous Moon. A gibbous Moon is one showing over half the surface. The Moon is said to wax when it is increasing in apparent size. The Moon's phases start with a new Moon, progress through to a full Moon via this gibbous phase, and then reduce to a new Moon again. One cycle takes around 28 days. The phases of the Moon are due to its orbit around Earth. When it lies in between the Earth and the Sun, only the unlit side faces us (new Moon). When it has moved round so that the Earth lies between it and the Sun, we see the full lit face, a full Moon. North is at top. For the sequence of the Moon's phases, see images R340/526-532.