Moon Phase Calculator
Enter any date to find the moon phase name and illumination percentage.
How It Works
Enter a date. The calculator converts it to a Julian Day Number, subtracts the reference new moon (Jan 6, 2000), divides by the synodic month (29.53059 days), and takes the fractional part to determine the current phase.
Formula
Phase = ((julianDay - 2451550.1) / 29.53059) % 1
Frequently Asked Questions
What is a moon phase?
A moon phase describes how much of the Moon's illuminated surface is visible from Earth. The cycle runs from New Moon through Full Moon and back, taking about 29.53 days.
How accurate is this calculator?
It uses the synodic month formula accurate to within a few hours for dates within a century of the year 2000.