The Moon’s phases follow a continuous, predictable celestial cycle as it orbits Earth over roughly a month. The portion of its sunlit surface visible from Earth changes daily. One of the most striking phases occurs when the Moon appears exactly half-illuminated. This distinct phase, where precisely 50% of the visible lunar disc is glowing, has a specific and counter-intuitive name in astronomy.
The Astronomical Terminology
The phase commonly called a “half moon” is scientifically known as a Quarter Moon. This designation marks two specific points in the lunar cycle: the First Quarter and the Third Quarter (also called the Last Quarter). These phases are major milestones in the Moon’s journey around Earth. The First Quarter occurs about one week after the New Moon, and the Third Quarter occurs about one week after the Full Moon. These four primary phases divide the entire lunar cycle, known as a synodic month, into four roughly equal periods.
Why a Quarter Moon Looks Half
The name “quarter” refers to the Moon’s orbital position, not its visual appearance. When the Moon reaches the First Quarter phase, it has completed exactly one-quarter of its orbit around Earth since the New Moon. The Third Quarter phase means it has completed three-quarters of that orbit.
At these two points, the Sun, Earth, and Moon form a nearly perfect 90-degree angle in space, known as quadrature. This alignment causes an observer on Earth to see the Moon illuminated precisely from the side. Although the Moon is always half-lit by the Sun, we only see this precise 50% illumination during the quarter phases. This geometry ensures that the line separating the illuminated side from the dark side, called the terminator, appears straight across the lunar disc.
Recognizing the First and Third Quarters
Distinguishing between the First and Third Quarter moons requires observing which side is illuminated and noting the Moon’s movement through the sky. The key difference lies in whether the Moon is waxing (growing brighter toward the Full Moon) or waning (growing darker toward the New Moon). Noting which side of the visible disc is lit is the most reliable way to identify the phase without consulting a calendar.
First Quarter
For Northern Hemisphere observers, the First Quarter Moon has its right half illuminated. This is the waxing phase, meaning the illuminated portion grows larger each night. This phase is best viewed in the evening sky, typically rising around noon and setting around midnight.
Third Quarter
Conversely, the Third Quarter Moon (Last Quarter) shows its left half illuminated from the Northern Hemisphere. This is the waning phase, indicating the light is diminishing as the Moon moves toward the New Moon. This phase is most visible in the morning sky, typically rising around midnight and setting near noon.