The appearance of the Moon, known as its phase, is determined by the portion of its sunlit surface visible from Earth. The phases result from the Moon’s approximately 29.5-day orbit and the constant illumination it receives from the Sun. This phenomenon is not caused by the Earth’s shadow, which only occurs during a lunar eclipse. The changing phase is a continuous cycle that progresses as the geometric relationship between the Sun, Earth, and Moon shifts throughout the lunar month.
The Constant Half-Illumination and Orbital Path
The fundamental principle governing the lunar phases is that the Sun always illuminates exactly one half of the Moon’s spherical surface. Similar to Earth, the Moon has a “day side” and a “night side,” with the light coming from a single direction. This constant half-illumination is the unchanging factor in the cycle.
The specific phase we observe from Earth depends entirely on the angle at which we view this illuminated half as the Moon travels along its orbital path. When the Moon is positioned between the Earth and the Sun, for example, the illuminated side faces away from us, making the Moon appear dark, which is the New Moon phase. Conversely, when the Moon is on the opposite side of Earth from the Sun, we see the entire sunlit face, resulting in a Full Moon.
Between these two extremes, the Moon’s orbit constantly changes the fraction of the sunlit portion visible to an observer on Earth. This changing perspective creates the different shapes—from a thin sliver to a full disk—that we call the lunar phases. The shifting angle dictates whether we see a crescent (less than half lit) or a gibbous shape (more than half illuminated).
The Eight Principal Lunar Phases
The continuous progression of the Moon’s appearance is divided into a cycle of eight distinct, traditionally recognized phases. This cycle begins with the New Moon, where the Moon is not visible because its dark side faces Earth. Following this, the visible illuminated portion begins to grow, a period known as waxing.
The Waxing Crescent phase shows a small sliver of light, which then grows to the First Quarter, where exactly half of the Moon’s face is illuminated. Next is the Waxing Gibbous phase, where the visible light expands to more than half but less than full illumination. The cycle reaches its peak with the Full Moon, which displays the entire sunlit face.
After the Full Moon, the illuminated portion begins to shrink, or wane, starting with the Waning Gibbous. This is followed by the Last Quarter (or Third Quarter), which again shows exactly half the Moon lit, but on the opposite side from the First Quarter. The phase ends with the Waning Crescent, which is a shrinking sliver of light, before returning to the New Moon to restart the cycle.
How the Moon’s Rise and Set Times Affect Visibility
The phase of the Moon determines its position in the sky relative to the Sun, which directly dictates when it rises and sets. For instance, during the Full Moon, the Moon is opposite the Sun in the sky; consequently, it rises around sunset and sets around sunrise, making it visible throughout the night. This alignment allows its fully lit face to be seen against the dark night sky.
In contrast, the New Moon rises and sets at roughly the same time as the Sun because it is in the same part of the sky. Its presence in the daytime sky means it is completely lost in the Sun’s glare, making it virtually invisible.
Intermediate phases have predictable visibility patterns that fall between these two extremes. A First Quarter Moon rises around noon and sets around midnight, meaning it is visible in the late afternoon and the first half of the night.
Conversely, the Last Quarter Moon rises around midnight and sets around noon, making it visible in the latter half of the night and the morning sky. Because the Moon moves about 13 degrees in its orbit each day, its rise and set times shift later by approximately 50 minutes daily.