The appearance of the Moon changes predictably over the course of approximately one month, a phenomenon known as the lunar phases. These shifting shapes are not due to the Moon physically changing size, but rather a change in the angle at which we view the sunlit portion of its surface as it orbits the Earth. Since the Moon does not produce its own light, its visibility depends entirely on reflected sunlight. The amount of the illuminated side visible from Earth varies, defining whether the visible section is growing (“waxing”) or shrinking (“waning”).
The Core Answer: Waxing Means Getting Bigger
The term “waxing” describes the half of the lunar cycle where the visible illuminated area is progressively increasing in size. This growth begins immediately after the New Moon, a phase where the sunlit side faces away from Earth and the Moon is not visible. During the waxing period, the illuminated fraction grows each night as the Moon moves away from the Sun in its orbit. This period includes the Waxing Crescent, the half-lit First Quarter Moon, and the Waxing Gibbous phase. The entire waxing period culminates when the Moon reaches the Full Moon phase, where its entire face is visible from Earth.
Defining Waning
The term “waning” refers to the period where the visible illuminated portion of the Moon is decreasing in size. This shrinking begins immediately after the Full Moon, which marks the peak of lunar illumination. During the waning period, the amount of the sunlit surface diminishes each night. The cycle continues with the Waning Gibbous phase, followed by the Last Quarter Moon, where exactly half of the face is illuminated. The final stage before the cycle resets is the Waning Crescent phase, where the remaining sliver of light shrinks until the Moon disappears completely at the next New Moon.
The Full Lunar Cycle in Context
The entire lunar cycle, from one New Moon to the next, takes approximately 29.5 days, which is known as the synodic month. This cycle is divided into the waxing and waning periods, each lasting roughly two weeks. The eight primary phases of the Moon follow a set, predictable order. This sequence begins with the New Moon, progresses through the Waxing Crescent, First Quarter, Waxing Gibbous, and culminates in the Full Moon. It then reverses through the Waning Gibbous, Last Quarter, and Waning Crescent. This systematic sequence is a direct result of the Moon’s orbital motion around the Earth and the constant illumination from the Sun.
Identifying the Difference
For an observer in the Northern Hemisphere, a simple rule allows for easy distinction between waxing and waning phases. When the Moon is waxing, the illuminated portion appears on the right side of the lunar disk and grows toward the left. Conversely, when the Moon is waning, the illuminated portion is on the left side and shrinks toward the right. A helpful mnemonic device uses the shape of the crescent: if the curve forms a capital letter ‘D,’ it is Waxing (Developing). If the illuminated shape is like the letter ‘C,’ it is Waning (Contracting).