Moon Phase Calculator

The calculator finds how many days have passed since a known new moon, then divides by the length of the lunar cycle (29.53 days) to place the moon within its phase. From that position it derives the phase name, the moon's age in days, and the illuminated fraction of the disc.

In order: New Moon (dark), Waxing Crescent, First Quarter (half lit, growing), Waxing Gibbous, Full Moon (fully lit), Waning Gibbous, Last Quarter (half lit, shrinking), and Waning Crescent — then back to new. "Waxing" means growing brighter; "waning" means fading.

It is the fraction of the moon's visible disc that is lit by the sun. 0% is a new moon, 50% is a quarter moon, and 100% is a full moon. The percentage is the same worldwide on a given date, even though the moon looks flipped between the Northern and Southern Hemispheres.

It is accurate to within a few percent of illumination and reliably correct on the phase name for any date, past or future. It uses a mean lunar cycle, so it can differ by a few hours from precise astronomical tables, which account for the moon's slightly irregular orbit.

Pick today's date and the calculator shows the days until the next full moon and the next new moon. Full moons occur about every 29.5 days, so there is usually one each month — occasionally two, the second of which is called a blue moon.