halfmoon

Halfmoon, or half moon, is a term used to describe two related concepts. In astronomy, it denotes the Moon's phase when exactly half of the near side is illuminated and visible from Earth; in everyday language, it can also refer to any semicircular shape that resembles a half circle.

In lunar terms, the half moon occurs during the first quarter and the last quarter of the synodic month. At these times the Sun, Earth, and Moon form about a 90-degree angle, and roughly 50 percent of the Moon's near side is illuminated. The line between light and dark—the terminator—passes close to the middle of the disk. Observers see the half moon in the afternoon and early evening at the first quarter, and in the pre-dawn hours at the last quarter. The scientific term for this phase is first quarter or last quarter; “half moon” is common in casual speech.

Other uses include naming in hobbyist and design contexts. For example, the Halfmoon Betta is a variety of Betta splendens with a large, semicircular tail that spreads about 180 degrees. The term halfmoon is also used more generally to describe semicircular shapes in architecture, furniture, and branding, where the motif appears as a curved, flat, or rounded form reminiscent of a half circle.