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Moon Phase Calculator

Discover the moon phase, illumination percentage, and lunar age for any date you choose.

Result

Full Moon
98.92%
13.8 days

How it is calculated

The date (YYYY-MM-DD) is turned into a Julian Day Number at 00:00 UTC. The tool measures the days elapsed since a known new moon (6 January 2000, 06:14 UTC) and takes the remainder after dividing by the mean synodic month of 29.53058867 days; that remainder is the moon's age. The phase name comes from splitting the cycle into eight equal parts. Illumination is (1 − cos(2π × age ÷ 29.53)) ÷ 2 × 100. Because it uses an average month length, real phase times can differ by up to about a day, and the moment of day is not considered.

Worked example

Enter 2025-01-13. The days since the reference new moon, taken modulo 29.53058867, give an age of 13.8 days. The cycle fraction is 13.8 ÷ 29.53 = 0.47, so about 3.7 eighths in, rounded to the nearest phase, which is the full moon. Illumination: (1 − cos(2π × 0.47)) ÷ 2 = 98.92%. The result is Full Moon, 98.92% lit, age 13.8 days.

Try this example →

Common mistakes

  • Typing the date in another order such as 13/01/2025 or 01-13-2025. Only the year-month-day format is accepted, otherwise nothing is shown.
  • Expecting a precise astronomical event time. The tool uses an average cycle, so the phase for dates close to a quarter or full moon can be off by a day.
  • Expecting illumination to read exactly 100% at full moon. It changes smoothly with the age, so even on the full-moon day it is usually a little below 100%, as in this example.
  • Ignoring the time zone and hemisphere. The date is treated at 00:00 UTC, and the moon's tilt as seen from the southern hemisphere is mirrored.

About this calculator

This calculator finds the moon phase, illumination percentage, and moon age (days since the last new moon) for any date, based on the 29.53-day synodic month referenced to a known new moon in January 2000.

Astronomers and stargazers use it to plan observation nights around a dark, moonless sky; photographers time full-moon shots with it; gardeners follow moon-phase planting traditions; and it's also handy for anyone tracking lunar calendar events tied to a specific moon phase.

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Comments (1)

  • Editorial team

    For planning a dark-sky night, look for dates a few days either side of the new moon, where illumination stays low. Note the tool works from an average cycle and treats each date at 00:00 UTC, and it does not tell you when the moon rises or sets, which matters as much as the phase. Check an almanac for the exact times. What do you plan to shoot or observe?

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