Moon Phase Calculator
Approximate phase name, illumination, and age from a civil date using a simple synodic-month model. Not a rise/set almanac and not weather.
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How to estimate moon phase
- Enter a civil date as YYYY-MM-DD.
- Read the phase name, illumination percent, and age in days.
- The cycle length is a mean synodic month of about 29.530589 days.
- This is not photography advice and not a rise time. Nothing is uploaded.
Moon phase
Simple synodic-month illumination model
| Synodic month | About 29.530589 days from new moon to new moon |
|---|---|
| Illumination | Fraction of the disk that appears lit |
| Age | Days since the model new moon |
| Names | New, crescent, quarter, gibbous, full |
| Epoch | Fixed new-moon Julian day in the engine |
| Not included | Moonrise, moonset, libration |
What this moon page estimates
The page takes the Julian day of the date, finds the remainder in a mean synodic month, and maps that fraction to a phase name and an illumination from (1 - cos(2 pi fraction)) / 2. It is a simple cycle, not a full lunar theory.
Synodic month, illumination, and phase name
The synodic month is the new-moon to new-moon period. Illumination is not the same as rise time. Quarter names here are first quarter and last quarter. Sun times for a place are on sunrise and sunset.
How this page differs from NOAA sunrise
Moon mode does not take latitude or longitude. It only needs a date. Sunrise on the sibling page uses NOAA with coordinates. Do not treat this illumination as a photography exposure table.
Limits
Mean month only. Real new moons wander. Not a nautical almanac and not weather. CZNull does not receive the date. See the disclaimer.