How moon phases are calculated
The moon completes a full cycle of phases, called a synodic month, in about 29.53 days on average, as it orbits the Earth and reflects different amounts of sunlight from our point of view. Astronomers track this using a known reference new moon date and the average cycle length.
How this calculator works
The calculator measures the number of days between your chosen date and a known reference new moon (January 6, 2000), then takes the remainder after dividing by the 29.53-day synodic month to find the moon’s ‘age’ in its current cycle. That age is mapped to one of the eight standard phase names, and illumination percentage is approximated using a cosine curve that models how sunlight visible from Earth changes smoothly across the cycle.
This calculation uses the average synodic month length, which is accurate to within about a day for most purposes; for precise astronomical or navigational use, consult a dedicated ephemeris or astronomical almanac.