Moon Phase Calculator – Track the Moon's Cycle for Any Date
The Moon moves through eight recognizable phases roughly every 29.53 days — a period astronomers call the synodic month. This tool tells you exactly where in that cycle the Moon sits on any date you choose, past, present, or future, along with how much of its visible disk is lit and when the next major phase will occur.
How the calculation works
The calculator anchors every calculation to a well known reference New Moon that occurred on 2000-01-06 18:14 UTC. For any target date, it computes the number of days elapsed since that reference, then takes the remainder after dividing by the synodic month length to get the Moon's age — the number of days since the most recent New Moon. That age is then split into eight equal segments (about 3.69 days each) to classify the phase as New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, or Waning Crescent.
Illumination percentage— the fraction of the Moon's visible disk that sunlight is hitting — is estimated with a cosine-based approximation of the phase angle: illumination = (1 - cos(2π × age / 29.53)) / 2 × 100. This produces 0% at New Moon, 100% at Full Moon, and roughly 50% at both quarters, matching what you would observe by eye.
Waxing vs. waning
Waxing means the illuminated portion is growing night after night, moving toward a Full Moon. Waning means it is shrinking, moving back toward a New Moon. Knowing which direction the cycle is heading is useful for planning — for example, the days around a New Moon offer the darkest skies for stargazing or astrophotography, while a Full Moon is brightest for night-time outdoor activity.
Next New Moon and Full Moon
Because the synodic month length is fixed, the calculator can work forward from the Moon's current age to find exactly how many days remain until the age next reaches 0 (New Moon) or 14.77 days (Full Moon, half the cycle). This gives you an instant countdown without needing to page through an almanac.
Supermoons and micromoons
The Moon's orbit is elliptical, so its distance from Earth varies between roughly 356,500 km at its closest (perigee) and 406,700 km at its farthest (apogee). A Full Moon that happens to coincide with a close approach is popularly called a supermoon, appearing slightly larger and brighter; one near apogee is a micromoon. The distance shown here uses a simplified orbital approximation and is intended as an indicator, not a precise ephemeris value.
Monthly calendar view
Switch to the Monthly Calendar tab to see the phase and illumination percentage for every day of a chosen month in one table, with major phase days (New Moon, First Quarter, Full Moon, Last Quarter) highlighted. The full table can be exported as a CSV file for planning, journaling, or sharing.
This calculator uses a simplified synodic-month formula that is accurate to within about a day for most dates between 1900 and 2100. It does not account for small periodic perturbations in the Moon's real orbit, so treat exact phase-change times as close approximations rather than precise astronomical predictions.
Common uses
Photographers and astronomers use moon phase data to plan dark-sky sessions away from bright moonlight. Anglers, hunters, and gardeners following lunar calendars use it to time activity around the lunar cycle. Many people simply use it to answer everyday questions like "what phase was the Moon on my birthday?" or "will there be a Full Moon on my camping trip?"