Moon Phase Calculator

The Moon completes one orbit around Earth in approximately 29.53 days — the synodic month. During this cycle, it passes through 8 distinct phases based on how much of its illuminated side faces Earth. The New Moon occurs when the Moon is between Earth and the Sun (invisible). As it moves, we see a Waxing Crescent, then First Quarter (half lit), Waxing Gibbous, Full Moon (fully lit), Waning Gibbous, Last Quarter, and finally Waning Crescent before returning to New Moon. The exact phases depend on the Moon's orbital position. Our calculator uses a known new moon reference point (January 6, 2000) and the precise 29.53-day cycle to calculate the phase for any date. Illumination is calculated using a cosine approximation of the angular relationship between Earth, Moon, and Sun.