The Moon’s appearance changes as it orbits Earth, creating what are known as the lunar phases. This cycle occurs because the Moon does not produce its own light, but instead reflects sunlight. The portion of the Moon’s sunlit half that is visible from Earth shifts depending on the relative positions of the Sun, Earth, and the Moon. This repeatable process takes approximately 29.5 days to complete, moving through eight recognized phases.
Defining the Waning Phase
The term “waning” describes the portion of the lunar cycle when the visible, illuminated area of the Moon decreases in size. This phase begins immediately after the Full Moon, when 100% of the visible surface is lit, and continues until the Moon reaches the New Moon phase. The entire waning period lasts about two weeks, or approximately half of the complete lunar month.
For observers in the Northern Hemisphere, the waning phase is identified by the location of the light and shadow. As the illumination shrinks, the sunlit portion appears on the left side of the lunar disk. Conversely, the right side is the first to fall into shadow as the Moon progresses away from the Full phase. This visual characteristic indicates that the illuminated area is getting smaller.
The Three Stages of Waning
The decrease in illumination from the Full Moon to the New Moon is tracked through three distinct stages. The first stage is the Waning Gibbous, which occurs immediately after the Full Moon. During this time, the Moon remains more than half illuminated, appearing bulbous or “gibbous,” but the visible light is shrinking each night. The Waning Gibbous stage transitions into the next phase when the illumination reaches exactly 50%.
The second stage is known as the Last Quarter Moon, sometimes referred to as the Third Quarter Moon. At this point, precisely half of the Moon’s face is illuminated, marking the halfway point of the entire waning process. This half-lit appearance is the opposite of the First Quarter Moon, as the left half is illuminated in the Northern Hemisphere.
Following the Last Quarter, the Moon enters the final stage of the cycle, the Waning Crescent. During this phase, less than half of the Moon is visible, appearing as a thin, curved sliver of light. The crescent shrinks daily until the Moon becomes completely invisible at the New Moon phase.
Observing the Waning Moon
The waning phases are primarily visible during the late night and morning hours due to the Moon’s position relative to the Sun in its orbit. The Waning Gibbous phase, which follows the Full Moon, rises relatively late at night but is visible during the early morning. As the orbit continues, the Moon rises later each night.
The Last Quarter Moon rises around midnight and remains visible high in the sky until it sets around noon the following day. This makes the Last Quarter easy to observe in the daylight sky, as it is positioned at a 90-degree angle from the Sun. The final stage, the Waning Crescent, rises after midnight and is best viewed just before sunrise, low on the eastern horizon in the pre-dawn sky.