Find the moon's phase and approximate illumination percentage for any date using the synodic (lunar) month cycle.
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The moon's phase on any given day depends on how far it has progressed through its roughly 29.53-day synodic month — the average time between one new moon and the next, as observed from Earth. This calculator counts the number of days elapsed since a well-documented reference new moon (January 6, 2000) and finds the remainder after dividing that count by the synodic month length, which gives the moon's current "age" at any point within its ongoing cycle. That age value is then used to classify the current phase into one of eight commonly named stages, running from new moon through the waxing and waning quarters to full moon and back again, and also to estimate the percentage of the moon's visible disk that appears illuminated, using a standard cosine-based approximation of how sunlight falls across the lunar surface. Because the real lunar cycle length varies slightly from month to month rather than staying perfectly fixed, results can drift by up to roughly a day near the boundaries between named phases.
Every phase of the moon you've ever seen — from a barely-there sliver to a fully lit disk — comes down to one simple, repeating cycle: the changing angle between the Earth, the moon, and the sun as the moon orbits us roughly once a month. This calculator works out where in that cycle the moon sits on any date you choose.
The cycle in question is called the synodic month, and it averages about 29.53 days — slightly longer than the moon's actual orbital period around the Earth, because the Earth itself is also moving around the sun during that time, requiring the moon to "catch up" a bit further each cycle. By counting the number of days since a well-documented past new moon and finding the remainder after dividing by 29.53, this calculator determines exactly how far into the current cycle any given date falls.
That position in the cycle, expressed as a fraction from 0 (new moon) to 1 (back to new moon again), maps directly onto the eight named phases most people are familiar with: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent. "Waxing" phases occur during the first half of the cycle as the visible illuminated portion grows, while "waning" phases occur during the second half as it shrinks back toward new moon.
The illumination percentage shown alongside the phase name is calculated using a standard cosine-based formula that models how the illuminated fraction changes smoothly rather than in sudden jumps — it's roughly 0% at new moon, rises smoothly to 100% at full moon, then falls back down. Because the real lunar cycle isn't perfectly uniform in length from month to month (it's affected by the moon's elliptical orbit), an estimate based on a single average cycle length can drift by up to roughly a day over time, so treat the exact phase-boundary date as approximate rather than to-the-hour precise.
It's generally accurate to within about a day, since it uses a fixed average cycle length rather than modeling the small month-to-month variations in the moon's actual orbit.
Moon age is simply how many days have passed since the most recent new moon in the cycle, ranging from 0 (new moon) up to just under 29.53 (the next new moon).
Each named phase (like 'full moon') actually represents a period around the exact peak or midpoint of the cycle where the moon looks essentially the same to the naked eye, not a single instant.
No — unlike sunrise and sunset times, the moon's phase is the same worldwide on a given date (though moonrise and moonset times themselves do vary by location).