The Moon’s phases are a continuous, predictable cycle of change in the amount of its illuminated surface visible from Earth. This cycle, which takes approximately 29.5 days to complete, results from the Moon’s changing position in its orbit around our planet. We see the Moon transition from total darkness to full illumination and back again. The term “waning” describes the half of this process when the Moon’s brightness begins to visibly diminish after reaching its peak.
Defining Waning vs Waxing
The word “waning” comes from an Old English term meaning to decrease or shrink. Waning refers to the stage of the lunar cycle where the visible illuminated portion of the Moon is decreasing each night. This phase spans from immediately after the Full Moon up until the New Moon.
This process is the opposite of “waxing,” which is when the illuminated surface is growing in size, moving from the New Moon toward the Full Moon. A simple visual cue can help an observer in the Northern Hemisphere distinguish between the two: if the light appears to be on the left side of the Moon’s disk, it is waning. If the light is on the right side, it is waxing. The illuminated portion gets visibly smaller night after night as it progresses toward the New Moon.
The Waning Phases Explained
The entire waning period is subdivided into three distinct phases, each defined by the amount of the Moon’s surface that remains lit. The cycle begins immediately after the Full Moon with the waning gibbous phase. During this time, more than half of the Moon’s face is illuminated, but the bright area is visibly shrinking from the right edge.
The next phase is the Third Quarter Moon, sometimes referred to as the Last Quarter Moon. At this point, exactly half of the Moon’s visible disk is illuminated, appearing as a perfect half-circle. The Moon has now completed three-quarters of its orbit around the Earth.
Finally, the cycle enters the waning crescent phase, where less than half of the disk is lit, appearing as a thin sliver of light on the left side. This crescent continues to thin until it disappears, leading into the New Moon. The waning crescent is the final visible phase before the cycle restarts.
The Astronomical Cause of Waning
The appearance of the waning Moon is not caused by a shadow from the Earth, but by the changing angle at which we view the Moon’s sunlit surface. The Moon always has one half illuminated by the Sun, but our perspective on that illuminated half changes as the Moon orbits our planet. Waning occurs as the Moon moves through the second half of its orbit, from the position opposite the Sun back toward the Sun’s general direction.
After the Full Moon, the Moon continues its counter-clockwise path. From Earth’s vantage point, we begin to see more of the Moon’s dark side. Since the Moon is moving closer to the Sun in the sky from our perspective, less of its fully lit face is directed toward us. The decreasing illumination reflects a growing proportion of the Moon’s night side facing the Earth. Waning continues until the Moon is positioned between the Earth and the Sun, resulting in the New Moon.