How this works
Enter the values shown, review the result, and verify assumptions before acting. The calculation is performed on your device without uploading these entries.
Find approximate lunar age and illuminated fraction for a date. The input fields are labeled with the units or formats used in the calculation. Change one value at a time to see how the result responds, and compare the output with the worked example below.
Formula or method
The method above describes the calculation or algorithm being used by this browser utility. It is shown so you can understand the result, check assumptions and repeat the calculation independently if needed.
Worked example
The Moon is near full around half a synodic cycle after new moon.
Limitations and accuracy
Simplified model; astronomy ephemerides are needed for exact events.
The calculation runs locally in your browser. No result is guaranteed to be suitable for financial, legal, medical, or safety-critical decisions. Check independent authoritative sources in those cases.
More context
The synodic lunar cycle averages about 29.53 days, so moon phase categories move through calendar dates. Exact phase times differ by time zone: a full moon might occur on one date in UTC but the previous date in another region. Simple calculations are useful for planning and learning but may differ from precise astronomical ephemerides. Moonrise and moonset also depend on coordinates, elevation and refraction.