The Moon does not generate its own light; it shines by reflecting sunlight, and half of its sphere is always lit by the Sun. The changing appearance of the Moon throughout the month is a result of the Moon’s orbit around Earth and the angle at which we view its illuminated surface. As the Moon completes its approximately 29.5-day cycle, the alignment of the Sun, Earth, and Moon determines how much of that sunlit portion is visible to us. These predictable shifts in appearance are known as the lunar phases.
Defining the Gibbous Phase
The term “gibbous” comes from a Latin word meaning “hump,” which accurately describes the shape of the Moon during this phase. A gibbous moon occurs when the illuminated portion of the Moon’s face appears larger than a semicircle but is not yet fully a circle. Specifically, this phase represents the period when more than 50% but less than 100% of the Moon’s disk is visible from Earth. The gibbous shape appears because the Moon has moved far enough in its orbit that the angle between the Sun, Earth, and Moon allows us to see a substantial, bulging amount of the sunlit side. The gibbous phase is one of the four intermediate phases that bridge the appearance of the half-lit quarter moons and the fully illuminated full moon.
Waxing and Waning Gibbous
The gibbous phase is divided into two parts, depending on whether the illuminated area is growing or shrinking. The waxing gibbous occurs after the First Quarter moon and continues until the Full Moon. The term “waxing” means increasing, indicating that the visible illuminated area is growing larger each night. During this stage, the Moon is moving from being half-illuminated to fully illuminated, and the right side appears bright for observers in the Northern Hemisphere. Following the Full Moon, the Moon enters the waning gibbous phase, which lasts until the Third Quarter moon. “Waning” means decreasing, signifying that the illuminated portion is visibly shrinking each night. This phase begins immediately after the Moon reaches 100% illumination and concludes when it returns to 50% illumination. During the waning gibbous, the left side appears bright in the Northern Hemisphere.
Observing the Gibbous Moon
The gibbous phase is one of the longest in the lunar cycle, lasting for approximately 6 to 7 days on either side of the Full Moon. The best time to view the Moon depends on whether it is waxing or waning. The waxing gibbous Moon is often most noticeable in the evening sky, as it rises in the afternoon and remains visible until after midnight. In contrast, the waning gibbous Moon is primarily seen late at night and into the morning hours. It typically rises around the time the Full Moon sets and remains visible high in the sky until it sets near noon the following day.