A Moon phase describes the changing illuminated appearance of the Moon as seen from Earth. The primary named phases are New Moon, First Quarter, Full Moon, and Last Quarter. The appearance changes continuously between them; a calendar records the calculated instants of those milestones.
At a quarter phase, roughly half of the visible lunar disk is illuminated. “Quarter” refers to the phase cycle, not to one quarter of the disk being bright. A phase is also different from a Moon sign, which describes a zodiac position.
A phase timestamp does not specify local moonrise, weather, or visibility. Use the Moon phase calendar for dated UTC entries and the calendar reading guide to interpret the timezone and limits of those entries. The methodology records the calculation convention; NASA’s Moon phase explanation illustrates the changing appearance.