The moon’s appearance changes in a predictable, repeating sequence known as the lunar cycle, which lasts approximately 29.5 days. This cycle is divided into eight distinct phases based on the moon’s position relative to the Earth and the Sun. The Waning Gibbous phase represents a specific transitional period where the moon’s visibility is decreasing as it moves through the second half of its monthly journey.
Defining the Waning Gibbous Phase
The Waning Gibbous phase describes the specific shape of the moon when more than half of its visible disk is illuminated, but that illuminated fraction is shrinking, beginning immediately after the Full Moon and continuing until exactly half of the moon is lit. During this period, the moon’s illumination decreases from 99.9 percent to 50.1 percent as it orbits Earth.
Visually, the shape of the moon during this phase is distinctly non-circular, appearing bulging or convex on the illuminated side. For observers in the Northern Hemisphere, the illuminated portion is seen on the left side, with the dark terminator line gradually moving across the disk toward the left. The subtle reduction in light is not immediately noticeable on the first night, making the initial Waning Gibbous moon look almost identical to the Full Moon.
Placement in the Lunar Cycle
This phase holds the sixth position within the lunar cycle, marking the beginning of the second half of the month. It commences the moment the moon passes the Full Moon alignment and starts its path back toward the Sun. The entire Waning Gibbous phase lasts about 7 days, or roughly one quarter of the moon’s complete orbital period.
The transition from the fully illuminated Full Moon to the half-illuminated Third Quarter Moon defines the boundaries of this phase. As the moon continues its orbit, the angle between the Earth, Moon, and Sun changes, causing less of the sunlit surface to be visible. Following the Waning Gibbous, the moon reaches the Third Quarter phase, after which it becomes a Waning Crescent.
The Meaning of Waning and Gibbous
The name “Waning Gibbous” is a precise astronomical description derived from two distinct terms clarifying the moon’s appearance and change. The word “gibbous” comes from the Latin term gibbus, which translates to “humped” or “hump-backed,” referring to the convex shape that is greater than a semicircle. This term is used to describe the moon when its illuminated area exceeds the halfway point.
The term “waning” signifies a reduction or decrease in the visible illuminated area, indicating that the moon is shrinking in apparent size. This directly contrasts with the “waxing” phases, where the illuminated portion is growing larger each night. The combination of these two words perfectly encapsulates the phase: a bulging shape that is actively lessening in visible light.
Visibility and Observation Timing
The Waning Gibbous moon is best observed during the late night and early morning hours because, following the Full Moon which rises near sunset, it rises progressively later each night. On the first night of the phase, it rises shortly after sunset, but by the end of the week, it rises closer to midnight.
Consequently, this phase is often visible high in the sky during the pre-dawn and morning hours. It is not uncommon to see the brightly illuminated Waning Gibbous moon setting in the western sky well after the sun has risen. The precise timing of the moonrise and moonset shifts by about 50 minutes each day as the moon continues its orbit around the Earth.