The terms “waxing” and “waning” describe the fundamental pattern of cyclical change: the process of increasing and decreasing over time. While most commonly associated with the Moon, they serve as a simple binary concept for any natural system that grows toward a peak and then recedes. Understanding the difference between these two actions is key to interpreting various natural phenomena, from the night sky to biological rhythms. The distinction lies entirely in which direction the process is moving: toward growth or toward decline.
Understanding the Terms: Growth vs. Decline
The verbs “wax” and “wane” are ancient, with roots in Old English that translate to “to grow” and “to diminish” or “to lessen.” Waxing describes a period where a quantity, intensity, or size is increasing or expanding. For example, a person’s influence might be described as waxing when it is expanding and gaining strength.
Waning describes the opposite condition: a period of decline, reduction, or contraction. If a light source or an individual’s energy is becoming dimmer or weaker, it is waning. These terms describe any natural cycle of growth and recession, establishing a foundation for describing patterns of increase followed by decrease. While most often heard in an astronomical context, they can also describe the strength of a sound, the popularity of a trend, or the intensity of an emotion. The core meaning remains the same: waxing is moving toward a maximum, while waning is moving toward a minimum.
The Primary Context: Waxing and Waning in the Lunar Cycle
The most frequent application of these terms is describing the appearance of the Moon’s illuminated surface as viewed from Earth over the approximately 29.5-day lunar cycle. The phases are a result of our changing perspective of the sunlit portion of the Moon as it orbits our planet. The waxing phase is the period of approximately two weeks between the New Moon and the Full Moon, when the visible illuminated area grows larger each night.
This growth begins immediately after the New Moon, where the Moon is invisible, and progresses through the waxing crescent phase, the First Quarter, and the waxing gibbous phase. During the waxing period, the terminator—the line separating light and shadow—moves across the Moon’s face, revealing more of the sunlit side. For an observer in the Northern Hemisphere, the illuminated part of the Moon appears to grow from the right side.
The cycle shifts to the waning phase the moment the Moon passes Full, beginning its two-week journey back toward the New Moon. During this time, the illuminated portion visible from Earth appears to shrink nightly. This recession moves through the waning gibbous phase, the Last Quarter, and the waning crescent phase.
As the Moon wanes, the shadow encroaches upon the visible light until the Moon is once again completely dark at the New Moon. In the Northern Hemisphere, the light appears to retreat from the left side, leaving the right side in shadow.
Visual Cues and Memory Tools for Identification
Observing the direction of the Moon’s curve provides a simple visual cue for determining whether it is waxing or waning. In the Northern Hemisphere, a helpful mnemonic involves comparing the Moon’s illuminated curve to the letters of the alphabet. If the illuminated portion resembles the letter “D,” it is a waxing phase, as it is “developing” or increasing its light toward the Full Moon.
Conversely, if the illuminated shape resembles the letter “C,” it is a waning phase, as the light is “concluding” or decreasing toward the New Moon. A common saying to remember this is “DOC,” where “D” is the shape of the First Quarter (waxing), “O” is the Full Moon, and “C” is the shape of the Last Quarter (waning). This visual trick is reliable because the sunlit portion always expands from the right and contracts from the left when viewed from the Northern Hemisphere.
The visual cues are reversed for observers in the Southern Hemisphere, where the illuminated portion appears to grow from the left and shrink from the right. In this case, the “C” shape indicates a waxing phase, and the “D” shape indicates a waning phase. These identifiers allow for quick recognition of the Moon’s current stage without needing to consult a calendar.