The Moon’s appearance changes nightly as it orbits our planet, creating a predictable sequence of shapes known as the lunar phases. These shifts result from the varying angles at which we view the Sun’s light reflecting off the Moon’s surface. The entire lunar cycle takes approximately 29.5 days to complete. Astronomers use the terms crescent and gibbous to distinguish the two primary categories of illumination.
Defining the Shapes: Crescent vs. Gibbous
The fundamental difference between a crescent moon and a gibbous moon lies solely in the amount of the Moon’s face that appears illuminated from our perspective on Earth. The phases are defined by a 50% illumination threshold, corresponding to the quarter phases. A moon is classified as a crescent when less than half (between 0% and 50%) of its visible surface is lit by the Sun. This thin, curved shape resembles a narrow sliver in the night sky. Conversely, the term gibbous describes the Moon when more than half (between 50% and 100%) of its surface is illuminated. The word “gibbous” comes from a Latin root meaning “hump-backed,” describing the Moon’s bulging, nearly full appearance. The percentage of illumination is the same globally, independent of location on Earth.
The Full Cycle Context
These two shapes are part of a continuous, repeating sequence of eight major phases that occur over the 29.5-day lunar month. The cycle begins with the New Moon, which is not visible because the sunlit side faces away from Earth. Following the New Moon, the illuminated portion grows through the Waxing Crescent phase until it reaches the First Quarter (50% visible). The Moon then enters the Waxing Gibbous phase, growing until it culminates in the Full Moon, when 100% of the visible side is lit. After the Full Moon, the process reverses, passing through the Waning Gibbous phase and then the Third Quarter. The cycle concludes with the Waning Crescent phase before the Moon becomes new again. Crescent phases flank the New Moon, while gibbous phases flank the Full Moon.
Waxing vs. Waning: Observational Differences
To fully identify a phase, the terms crescent and gibbous must be combined with either “waxing” or “waning,” referring to the direction of the change in illumination. Waxing means the illuminated portion is increasing, moving toward the Full Moon. Waning means the illuminated portion is decreasing, moving toward the New Moon. In the Northern Hemisphere, an observer can determine the direction of change by noting which side of the Moon is illuminated. A waxing moon will always appear lit on the right side, as the light “grows” toward fullness. Conversely, a waning moon will be illuminated on the left side. For example, a Waxing Crescent is a thin sliver of light on the right, while a Waning Crescent is on the left.