halfmoons

Halfmoons, also known as half moons or half-moon phases, refer to the Moon when approximately half of its visible disk is illuminated by the Sun as seen from Earth. In astronomy, these phases are more precisely called the first quarter and the last quarter (or third quarter). The term reflects their position in the lunar cycle, which lasts about 29.53 days, the synodic month. At the first quarter, the right half of the Moon is illuminated (in the Northern Hemisphere), and the Moon rises around noon and sets around midnight. At the last quarter, the left half is illuminated, and the Moon rises around midnight and sets around noon.

The visible boundary between light and dark, the terminator, appears as a relatively straight line near quadrature, giving the familiar semicircular appearance of the Moon at these phases. Because of lunar libration, slightly more than half of the Moon’s surface may be visible from some viewpoints, allowing small regions near the limb to come into view or disappear over time.

Observation timing and orientation: Halfmoons are readily visible in the evening sky during the first quarter and in the morning sky during the last quarter. The orientation of the illuminated limb depends on the observer’s latitude and season, so the crescent-like appearance of the illuminated side can vary.

Terminology and context: The term "half moon" is common in everyday language, but astronomers more often use the terms "first quarter" or "last/third quarter." Halfmoons have practical use in calendars and have featured in various cultural motifs and observational guides.