Find the Moon’s phase and illumination for any date.
Calculated instantly in your browser.
How is the Moon's phase calculated for a date?
The phase comes from the Moon's age — the days elapsed since the last new moon — within the 29.53-day synodic month, and the illumination percentage is derived from that age. Estimated from a known new moon, it is accurate to within about a day. For example, 21 January 2000 was a full moon, fully illuminated.
Understanding your result
The synodic month (new moon to new moon) averages 29.53 days. This calculator estimates the phase from a known new moon, accurate to within about a day; exact phase times vary slightly with location and time zone.
Formula and method
The phase comes from the Moon’s age — the days elapsed since the last new moon — within the 29.53-day synodic month. Illumination is derived from that age.
Assumptions and limitations
The phase is estimated from the average 29.53-day synodic month starting from a known new moon, so it is accurate to roughly a day rather than to the minute. The Moon's orbit is not perfectly regular, and exact phase times shift slightly with your location and time zone, which this simplified model does not fully resolve.
Worked example
21 January 2000 was a full moon, fully illuminated.
How to use this tool
- Enter the date.
- Read the phase and illumination.
- See the Moon’s age in days.
Common mistakes to avoid
- Expecting to-the-minute accuracy — the result is to within about a day.
- Assuming the phase is identical worldwide at the same clock time.
About the Moon Phase Calculator
The Moon Phase Calculator shows the phase of the Moon on any date — new, waxing, full or waning — along with the percentage illuminated and the Moon’s age in days.
Who should use this tool
Stargazers, photographers, gardeners, anglers and the curious.
Benefits
- Named phase with an icon.
- Illumination percentage.
- Moon age in days.
- Works for past and future dates.
Practical use cases
- Planning night photography around a full moon.
- Checking the phase for a special date.
- Timing stargazing for a dark, new-moon sky.
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Frequently asked questions
How accurate is it?
To within about a day. It uses the average synodic month from a known new moon, which is plenty for planning.
What is the Moon’s age?
The number of days since the last new moon, from 0 (new) up to about 29.5 (just before the next new moon).
Why might the exact phase time differ from an almanac?
This tool uses the average length of the synodic month, whereas the Moon's real orbit speeds up and slows down slightly. Almanacs also quote phase moments in a specific time zone. Together these mean the calculated phase can differ from a precise ephemeris by up to about a day.