Moon Illumination Calculator

Calculate the exact percentage of the Moon's surface illuminated on any date.

Select Date

How It Works

Enter a date. The tool computes the phase angle and applies the illumination formula to return the exact percentage of the lunar disc visible as lit.

Formula

Illumination % = (1 - cos(phase × 2π)) / 2 × 100

Frequently Asked Questions

How is moon illumination calculated?

Illumination uses the formula: (1 - cos(phase × 2π)) / 2 × 100. At new moon (phase=0) illumination is 0%; at full moon (phase=0.5) it is 100%.

What is 50% illumination?

50% illumination corresponds to the quarter moon phases (First Quarter or Last Quarter), when exactly half the lunar disc appears lit.