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