The confusion between the Moon’s size and its phases is common, often stemming from the specific terminology used to describe the lunar cycle. When observing the Moon over the course of a month, its appearance changes dramatically, leading many to wonder if the celestial body is actually growing or shrinking. These changes in visibility are not related to the Moon’s physical size, but rather to the amount of its sunlit surface we can see from Earth. The terms used by astronomers, such as “waxing” and “waning,” simply describe the daily increase or decrease in the illuminated portion of the Moon’s face.
The Direct Answer: Defining Waning
A Waning Moon is one where the visible, illuminated portion is decreasing each night, meaning it is getting smaller. The term “waning” comes from an Old English word meaning to decrease or gradually fade. This period occurs during the second half of the lunar cycle, between the Full Moon and the New Moon.
During the waning phase, the shadow on the Moon’s disk grows larger, gradually obscuring the visible light. For observers in the Northern Hemisphere, the illuminated side is on the left, and the shadow moves inward. This shrinking continues until the Moon disappears from view during the New Moon phase, completing the 29.5-day lunar cycle.
The Opposite: Understanding Waxing
The opposite is the waxing phase, where the illuminated portion of the Moon is increasing or getting bigger. A Waxing Moon occurs during the first half of the lunar cycle, starting after the New Moon and continuing until the Full Moon.
During the waxing period, reflected sunlight grows from a tiny sliver until the Moon’s entire face is lit. For Northern Hemisphere observers, the illuminated part expands from the right side. The term “waxing” means to grow or increase in size, describing the observed increase in brightness.
The Progression of Waning Phases
The waning phase begins right after the Full Moon, transitioning into the Waning Gibbous phase. In this stage, more than half of the Moon’s face is illuminated, but the bright area visibly shrinks nightly. The shape is described as “gibbous,” meaning humped or bulging.
This is followed by the Last Quarter Moon, sometimes called the Third Quarter Moon. At this point, exactly half of the Moon’s disk is illuminated, with the light appearing on the left side. The final stage is the Waning Crescent, where less than half of the Moon is visible as a diminishing sliver of light. This crescent shrinks until the Moon aligns with the Sun and Earth, resulting in the invisible New Moon.
Illumination vs. Physical Size
The Moon itself does not physically change size during the lunar cycle. Its diameter remains constant at approximately 2,159 miles, regardless of its phase. The perceived change, described by waxing and waning, is solely a result of the Moon’s position in its orbit relative to the Earth and the Sun.
The Moon does not produce its own light; we only see it because it reflects sunlight. As the Moon orbits the Earth, the angle at which we view its sunlit half constantly shifts. This changing geometry creates the illusion of a growing or shrinking disk by showing varying amounts of the sunlit and shadowed sides.