Waning half Moon. Optical image of a waning half Moon. The Moon is said to wane when it is diminis- hing in apparent size. This stage is known as the third quarter, and occurs 21 days into the cycle. The Moon's phases start with a new Moon, progress to the first quarter (a waxing half Moon), reach a full Moon, and then continue through to a new Moon again via a third quarter. 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, the unlit side faces us (a new Moon). When the Earth lies between it and the Sun, we see a full Moon. North is at top. For a sequence of the Moon's phases, see R340/562-567.