Find the moon phase, illumination percentage, and phase name for any date. Essential for planning dark-sky observing sessions.
Enter any calendar date (year, month, day). The calculator converts it to a Julian Day Number and counts the days elapsed since a reference new moon, modulo the 29.53-day mean synodic month.
The phase value runs from 0 (new) through 0.5 (full) and back, and illumination follows a cosine curve: (1 − cos(2π × phase)) / 2, so a first-quarter Moon at phase 0.25 shows 50% of its disk lit.
One of eight phase names is assigned by splitting the cycle into equal segments. Because the model uses the mean lunar cycle and ignores the Moon's elliptical orbit, results near a phase boundary can differ from ephemeris values by several hours.
Your date is converted to a Julian Day Number, and the days elapsed since a reference new moon are divided by the mean synodic month of 29.5306 days. The fractional remainder is the phase: 0 is new, 0.5 is full, and values between map to waxing or waning stages.
It is the fraction of the lunar disk that appears lit, modeled as (1 − cos(2π × phase)) ÷ 2. New moon gives 0%, the quarters give 50%, and full moon gives 100% — a 'quarter' moon is half-lit despite its name.
Moonlight brightens the sky and washes out faint nebulae and galaxies. Most deep-sky observers plan sessions around new moon, or keep them inside the window when the Moon is below the horizon; bright clusters and double stars tolerate moonlight far better.
It assumes every lunation lasts exactly the mean 29.5306 days, while real cycles vary by several hours because of the Moon's elliptical orbit. Near a phase boundary, the assigned name and the exact instant of full or new moon can differ slightly from almanac values.