The appearance of the Moon constantly shifts as it travels through its orbit. The common term for the phase where the Moon appears exactly split between light and dark is the “half moon.” This visual phenomenon occurs when exactly half of the Moon’s visible disk is illuminated by the Sun. Understanding this phase involves knowing the official astronomical terminology.
The Specific Names for a Half Moon
What is popularly called a half moon is known in astronomy as a Quarter Moon. This appearance happens twice during the Moon’s 29.5-day cycle: the First Quarter Moon and the Last Quarter Moon. The First Quarter Moon occurs when the illuminated portion is growing (waxing). The Last Quarter Moon, sometimes called the Third Quarter Moon, happens when the illuminated portion is shrinking (waning).
In both phases, the Moon is precisely 50% illuminated as seen from Earth. The distinction is which side is lit up. In the Northern Hemisphere, the First Quarter Moon appears with the right half illuminated, while the Last Quarter Moon shows the left half lit.
Why “Quarter” Means “Half” Visually
The confusing terminology arises because “Quarter Moon” refers to the Moon’s position in its orbit, not the amount of light we see. The Moon’s orbital path takes approximately 29.5 days to complete one full cycle of phases, which is divided into four main segments, or quarters.
The First Quarter Moon is reached when the Moon has completed the first quarter of its journey, about seven to eight days since the New Moon. At this point, the Moon is positioned 90 degrees away from the Sun when viewed from Earth. The Last Quarter Moon is reached after the Moon has completed three-quarters of its orbit, about 22 days into the cycle.
The term “Quarter” is a measure of time elapsed in the cycle, not the fraction of the disk that is visible to us. This astronomical definition refers to the Moon’s angular position relative to the Sun and Earth.
The Visual Appearance and Terminator Line
The appearance of a perfect half-circle is due to a feature known as the terminator. This dividing line separates the illuminated day side of the Moon from its dark night side.
During the Quarter Moon phases, the angle between the Sun, the Earth, and the Moon is exactly 90 degrees. This precise geometric alignment causes the terminator to appear as a perfectly straight line across the face of the Moon, creating the distinct visual effect of a half-illuminated circle.
In other phases, such as the crescent or gibbous phases, the terminator is curved. The long shadows cast along the straight terminator make lunar features, like craters and mountains, stand out in stark relief, making the Quarter Moons an ideal time for observation.