Moon Phase Calculator
The Moon goes through a complete cycle of phases roughly every 29.53 days — a period known as the synodic month. This calculator determines the lunar phase for any date you choose by computing how many days have elapsed since a known new moon (January 6, 2000) and expressing that as a fraction of the synodic cycle. From there it identifies the phase name, displays the corresponding emoji symbol, and estimates the percentage of the lunar disk currently illuminated.
Understanding moon phases has practical value beyond astronomy. Farmers have traditionally planted by the moon calendar, anglers use phase data to predict fish activity, and photographers plan outdoor shoots around the full moon. The eight standard phases — new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent — each mark a distinct visual appearance as the angle between the Sun, Moon, and Earth changes.
How it works
Age (days) = (target date − 2000-01-06 18:14 UTC) mod 29.53058867. Illumination % = round((1 − cos(2π × age / 29.53)) / 2 × 100). Phase is determined by the fractional position within the synodic cycle, divided into eight equal segments of ≈3.69 days each.
Use cases
- Planning stargazing or astrophotography sessions around the new moon
- Scheduling outdoor night events to coincide with the full moon
- Following traditional lunar gardening and planting calendars
- Predicting tidal patterns and optimal fishing windows
- Finding the moon phase on a historical date such as a birthday or anniversary
Frequently asked questions
How is the moon phase calculated for a given date?
The calculator counts the days since a known new moon (January 6, 2000) and takes that value modulo the synodic month of 29.53 days. The resulting age of the moon is expressed as a fraction of the cycle, which maps to one of the eight standard phases and an illumination percentage using the formula (1 − cos(2π × age / 29.53)) / 2.
How long is a full moon cycle?
A complete cycle from one new moon to the next, called the synodic month, lasts about 29.53 days on average. Dividing that period into eight equal segments gives each phase a duration of roughly 3.69 days. The slight variation from month to month is due to the Moon's elliptical orbit.
What are the eight phases of the moon in order?
The eight phases in order are new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent. Waxing phases occur as illumination increases toward the full moon, and waning phases as it decreases back toward the new moon. The full cycle takes about 29.53 days.
What moon phase was I born under?
Enter your birth date into the calculator and it computes the lunar age for that day, returning the phase name, illumination percentage, and emoji. Because the tool works entirely offline using the synodic cycle, it can find the phase for any past or future date. This is a popular way to discover your birth moon phase for keepsakes or astrology.
What does the illumination percentage mean?
The illumination percentage is the fraction of the Moon's visible disk lit by the Sun, from 0% at the new moon to 100% at the full moon. It is calculated as (1 − cos(2π × age / 29.53)) / 2 × 100, based on the Moon's position in its cycle. A first or last quarter, for example, shows about 50% illumination.