Moon Phase Calculator
See the Moon phase for any date (its name, illuminated percentage and age in days) plus the dates of the next new moon, first quarter, full moon and last quarter. Calculates in your browser.
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How it works
The Moon's phase is found from the angle between the Sun and the Moon as seen from Earth, giving the fraction of the disk that's lit and where in the roughly 29.5-day cycle the Moon is. Age counts the days since the last new moon.
A waxing Moon is growing toward full; a waning Moon is shrinking toward new. The four principal phases (new, first quarter, full and last quarter) are the cycle's quarter points. A date is named after a principal phase when that phase's exact moment falls on it in the zone you pick, as calendars do; other dates take the in-between name.
Phase times use Jean Meeus's lunar-phase method (Astronomical Algorithms, chapter 49) with the Earth-rotation correction ΔT. Checked against the U.S. Naval Observatory's published times for sample years from 1700 to 2100, every phase was within 2 minutes. Far outside those centuries the Earth's rotation is less well known, so times there are less certain. Everything runs locally in your browser.
What this does
A moon phase calculator shows the Moon’s phase for any date (its name, illuminated percentage, and age in days) plus the dates of the next principal phases.
How to use it
- Choose a date.
- Read the phase name and illumination.
- See the Moon’s age in days.
- Check the dates of the next new, quarter, and full moons.
Understanding your result
Illumination is the lit fraction of the visible disk, from 0 percent at new moon to 100 percent at full, and the age is days since the last new moon. Use the upcoming-phase dates to plan around dark skies for stargazing or a bright full night, remembering the cycle averages 29.5 days so the dates drift through the calendar month to month.
Example
A full moon shows 100% illumination and an age near 14–15 days.
Last updated .
Frequently asked questions
What does the illumination percentage mean?
It’s the fraction of the Moon’s visible disk that’s lit by the Sun, 0% at new moon, 50% at the quarters, and 100% at full moon.
What is the Moon’s age?
The number of days since the last new moon. A full lunar cycle averages about 29.5 days, so a full moon falls near day 14–15.
How precise are the phase dates?
The new, first-quarter, full and last-quarter times are calculated with Jean Meeus’s lunar-phase method plus the correction for the Earth’s slowing rotation (ΔT), and shown in your chosen time zone. Checked against the U.S. Naval Observatory’s published times for sample years from 1700 to 2100, every phase was within 2 minutes.
Is my data stored?
No. Everything is calculated locally in your browser.
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