Lunar Phase & Illumination Calculator
Calculate the approximate moon illumination percentage and lunar phase based on days elapsed since the last new moon.
Results
What is it?
For entertainment purposes only. The lunar cycle (synodic month) is approximately 29.53 days from new moon to new moon. The moon's illuminated fraction follows a cosine curve over that period. This calculator maps days since the last new moon to illumination percentage and one of eight traditional phase names. Actual illumination may differ slightly due to the moon's elliptical orbit.
How to use
Find the date of the most recent new moon (from a moon phase calendar or astronomy app), count the number of days from that new moon to today, and enter the value. The calculator returns the approximate illumination percentage and the phase index.
Example scenario
7 days since new moon: illumination � 47% � this is the First Quarter phase (waxing, half-lit). At 14�15 days, illumination reaches ~99�100%: Full Moon. At 22 days, it is approximately Last Quarter, dropping back to ~50% illumination but now waning.
Pro tip
The full moon is popular in astrology and folklore � many cultures tie planting cycles, fishing, and personal energy levels to lunar phases. Whether or not you believe in lunar effects, moon phase knowledge is practically useful: a bright full moon provides excellent natural light for night photography and outdoor navigation, while a new moon offers the darkest skies for stargazing.