Moon Phase Calculator

Free Moon Phase Calculator — find the Moon phase, age and illumination percentage for any date plus the upcoming new and full moons.

929.8K uses Updated · 2026-05-13 Runs locally · zero upload
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How to Use Moon Phase Calculator

  1. Pick a date — Past, present or future; the date is treated as 12:00 UTC.
  2. Read the result — The Moon Phase Calculator shows the phase name, moon age in days, illumination percentage and the next new and full moon dates.

Formula & Theory - Moon Phase Calculator

The Moon’s synodic period (one new moon to the next) is 29.530588853 days. Given the Julian Day JD of the target date and the Julian Day of a reference new moon JD0:

age          = ((JD - JD0) mod synodic + synodic) mod synodic
phaseFraction = age / synodic
illumination = (1 - cos(phaseFraction * 2 * PI)) / 2  (in [0, 1])

The phase name buckets phaseFraction into eight equally-sized regions:

0.000 - 0.062  New Moon
0.062 - 0.188  Waxing Crescent
0.188 - 0.312  First Quarter
0.312 - 0.438  Waxing Gibbous
0.438 - 0.562  Full Moon
0.562 - 0.688  Waning Gibbous
0.688 - 0.812  Last Quarter
0.812 - 0.938  Waning Crescent
0.938 - 1.000  New Moon

Next new moon is JD + (synodic - age); next full moon is JD + ((synodic/2 - age) mod synodic).

Use Cases for Moon Phase Calculator

The Moon Phase Calculator is useful for:

  • Astrophotography & stargazing — Plan deep-sky photography around new moons.
  • Fishing & gardening — Many traditions tie activity to the lunar cycle.
  • Festivals — Identify upcoming full moons such as the Mid-Autumn or Harvest Moon.
  • Education — Demonstrate phases to students using any date.
  • Travel — Decide whether your trip overlaps with a full moon.
  • Personal interest — Discover the phase on your birthday or any meaningful date.

Because the Moon Phase Calculator runs entirely in your browser, it works offline and never sends your queries to a server.

Frequently asked questions about Moon Phase Calculator

How accurate is the Moon Phase Calculator?

The Moon Phase Calculator uses the mean synodic month of 29.530588853 days and a reference new moon near 2000-01-06 UTC. Results are accurate to within a few hours, which is enough for everyday use and casual astronomy.

What is 'moon age'?

Moon age is the number of days since the most recent new moon. It is the basis of every phase: 0 = new moon, ~7.4 = first quarter, ~14.8 = full moon, ~22.1 = last quarter.

What does illumination percentage mean?

It is the fraction of the Moon's visible disc that is lit by the Sun. It ranges from 0% at a new moon to 100% at a full moon following a cosine curve over the lunar cycle.

Is my data stored?

No. All calculations happen in your browser; nothing is sent to a server.