What the Moon Phase Calculator does
This calculator gives the Moon's phase, the percentage of its disc that is lit and its age in days for any date and time, the exact times of the next new moon, first quarter, full moon and last quarter in your time zone, and a day-by-day table for the whole month.
Phase times come from the algorithms in Jean Meeus's Astronomical Algorithms (chapter 49), which reproduce published almanac times to within a minute or so; the lit fraction uses the book's chapter 48 method. Everything is calculated in your browser - no lookup, no live feed.
How to use it
- Pick a date and a local time, or press Now. The Moon's phase is the same for everyone on Earth at a given instant, so the time zone only affects clock times and which calendar day an event lands on.
- Read the phase name and the illuminated percentage. The small disc shows the lit side as seen from the northern hemisphere; from the southern hemisphere it appears mirrored.
- Check the table of the next four principal phases for exact local times - useful for night photography, fishing calendars, festival dates or observing plans.
- Scroll to the month table to see each day's phase at local noon and the exact phase events that fall on each date. Download it as CSV if you want it in a calendar.
Reading the results
New moon, first quarter, full moon and last quarter are instants, not days: they are the moments the Moon's ecliptic longitude is 0°, 90°, 180° and 270° from the Sun's. This tool names a day by one of them only within about half a day of the exact moment; otherwise it uses the in-between names - waxing crescent, waxing gibbous, waning gibbous and waning crescent.
Illuminated fraction is the share of the visible disc in sunlight. It is 50% at the quarters, not at the halfway points in time, and it changes slowly near full and new moon and fastest near the quarters.
Age is the time since the last new moon. A full lunation (synodic month) averages 29.53 days but varies by up to about 7 hours either way, because the Moon's orbit is elliptical and the Sun perturbs it - which is why the tool calculates each phase rather than adding a fixed period.
Worked example: the total solar eclipse of 8 April 2024
The Meeus method gives the April 2024 new moon at 18:21 UTC on 8 April - the same minute that almanacs publish, and the reason a solar eclipse happened that day: a solar eclipse needs a new moon close to one of the Moon's orbital nodes.
Choosing 13:00 in America/Chicago (18:00 UTC) on that date, the tool shows a new moon with essentially 0% illumination, about 29.4 days after the previous new moon of 10 March. The next full moon follows on 23 April at 23:49 UTC, 15.2 days later.
Changing the zone to Asia/Tokyo moves the new moon to 03:21 on 9 April local time - same instant, different calendar day - which matters if you are dating an event by phase.
Formulas and scoring rules
- Mean phase
JDE = 2451550.09766 + 29.530588861 k + 0.00015437 T^2 - ...k is an integer for new moons, +0.25 first quarter, +0.5 full, +0.75 last quarter (Meeus 49.1).- Corrections
JDE += periodic terms in M, M', F, Omega + 14 planetary termsSeparate sets for new/full and for the quarters, with the W correction at the quarters.- Time scale
UT = TT - Delta-TDelta-T from the Espenak-Meeus polynomial, about 70 s in the 2020s.- Illuminated fraction
k = (1 + cos i) / 2i is the phase angle from the Moon's mean elongation and anomalies (Meeus 48.4, low precision; about 0.002 accuracy).
Limitations: what the result does not prove
- Moonrise and moonset are not calculated. They depend on your exact location and the Moon's rapidly changing declination and distance.
- The disc drawing is schematic. It shows how much is lit and on which side, not the Moon's tilt at your latitude or its libration.
- Illumination is a low-precision estimate, good to about a fifth of a percentage point - fine for planning, not for research.
- Religious and civil calendars that begin months at the first sighting of the crescent depend on local visibility, weather and the authority's rules, which this tool does not model.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- US Naval Observatory - Phases of the Moon and percent of the Moon illuminated
- NASA - Moon phases
- NASA Eclipse Web Site - Delta T (Espenak and Meeus)
Frequently asked questions
What percentage of the Moon is lit at first quarter?
About 50%. First quarter is named for the Moon being a quarter of the way round its orbit, not a quarter lit. At that instant the Sun, Earth and Moon form a right angle and we see half of the sunlit hemisphere, so half of the visible disc is bright.
Is the full moon on the same date everywhere?
The instant is the same, but the calendar date can differ. A full moon at 02:00 UTC is on the previous evening in the Americas and in the morning in Asia. Set your own time zone and the tool shows the date and time as you would see it on a local calendar.
How long is a lunar month?
The synodic month - new moon to new moon - averages 29.530589 days, about 29 days 12 hours 44 minutes. Individual months range from roughly 29.27 to 29.83 days, because the Moon speeds up and slows down on its elliptical orbit.
What does waxing and waning mean?
Waxing means the lit part is growing, from new moon to full moon; waning means it is shrinking, from full back to new. In the northern hemisphere a waxing Moon is lit on the right-hand side and a waning Moon on the left; south of the equator it is the other way round.
Why do different websites give slightly different moon phase times?
Most differences are a minute or two, from rounding, from using Terrestrial Time instead of Universal Time (they differ by about 70 seconds), or from simplified formulas. Larger differences usually mean a site is using a fixed 29.5-day cycle rather than calculating each phase.
What is the moon age in days?
It is the time since the most recent new moon. A 7-day-old Moon is near first quarter, a 15-day-old Moon is near full. Because the cycle length varies, age is a rough guide to phase; the illuminated percentage is the more direct measure of what you will see.
Last reviewed by the A2Z.Tools team against the sources listed above.
