Moon Phase Calculator
Find the moon phase for any date: phase name (New Moon, First Quarter, Full Moon…), illumination percentage, moon age in days, and the next new and full moons — with a rendered moon-disc that flips for the Southern Hemisphere. Runs entirely in your browser.
Input
Pick any date between 1900 and 2100 to see its moon phase.
Flips the moon disc to match the side of the Earth you live on.
Output
The moon disc will appear here.
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Guides
The Moon Phase Calculator tells you what the moon looked like — or will look like — on any date between 1900 and 2100. Enter a date, and it reports the phase name (New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, or Waning Crescent), the percentage of the disc that is illuminated, the moon's age in days since the last new moon, and the dates of the next new and full moons. It also draws the moon disc as a crisp, scalable SVG so you can see the exact sliver of light at a glance.
How to use it
- Pick a date. The calculator defaults to showing the phase for whatever date you enter and updates instantly as you change it.
- Choose your hemisphere. In the Northern Hemisphere a waxing moon lights up on the right; in the Southern Hemisphere the view is mirrored. The View From option flips the rendered disc so it matches the sky where you actually are.
- Read the results. The phase details table lists the phase name, illumination, moon age, position in the synodic cycle, the next new and full moon, and the zodiac constellation the moon is passing through. You can copy the details as CSV or download the moon-disc SVG.
How the calculation works
The tool models the lunar cycle as a steady synodic month of 29.530588853 days — the average time from one new moon to the next — measured from a known reference new moon (Julian Date 2451550.1, on 6 January 2000). The fraction of the cycle elapsed for your date gives the phase; the illuminated fraction is derived from that phase with a cosine curve, so it climbs smoothly from 0% at new moon to 100% at full moon and back. The moon's zodiac sign comes from its mean ecliptic longitude.
Because it uses a mean cycle rather than a full ephemeris, the phase names and illumination are accurate to within a few percent — perfect for planning, photography, gardening by the moon, or satisfying curiosity, but not a substitute for a precise astronomical almanac. All dates and math are computed in UTC, so the result is the same no matter where the calculation runs.
Why is the illumination not exactly 0% or 100%?
The real moon's orbit speeds up and slows down slightly, so a "new" or "full" moon rarely falls exactly at midday on the date you pick. A reading of 0.01% or 99.98% simply means you are a few hours away from the precise moment of the phase.
What does "moon age" mean?
Moon age is the number of days since the most recent new moon. A brand-new moon is 0 days old; a full moon is roughly 14.8 days old; the cycle completes at about 29.5 days, when the next new moon begins.
Does it work for past and historical dates?
Yes. Any date from 1900 to 2100 works the same way, so you can look up the moon phase for a birthday, a wedding, a historical event, or a future trip.
Privacy
Everything runs in your browser. The date you enter never leaves your device.