The moon runs through its phases every 29.53 days on average, but the exact moments of new and full moon wobble by many hours from one month to the next. This calculator works from the real lunar orbit rather than a simple average. Enter a date and time and it reports the phase name for that day, the percentage of the disk that is lit, the moon’s age, and when the next full and new moons happen in your time zone.
How to use the moon phase calculator
- Choose the date and the local time you care about (evening is the default).
- Pick your time zone as a UTC offset. In summer, use your daylight-time offset (US Eastern is UTC−4 from March to early November).
- Choose Northern or Southern Hemisphere so the drawing shows the correct side lit.
- Read the phase, illumination and moon age on the tape. The table lists the next eight principal phases.
How the phase is calculated
A synodic month — new moon to new moon — averages 29.530589 days. Dividing the time since the last new moon by that length gives a rough position in the cycle:
That estimate can be off by several percentage points, because the moon moves faster near perigee and slower near apogee. The calculator instead:
- finds the instants of the principal phases with Meeus’s algorithm (a mean lunation plus about 40 periodic corrections for the Sun’s and moon’s orbital eccentricity);
- computes the phase angle — the Sun–moon–Earth angle — and from it the illuminated fraction, k = (1 + cos i) ÷ 2.
The day is named the way almanacs do it: if a new moon, first quarter, full moon or last quarter happens during that local calendar day, the day takes that name. Between them the moon is a waxing crescent, waxing gibbous, waning gibbous or waning crescent.
Worked example
October 21, 2026, 9:00 p.m., US Eastern Daylight Time (UTC−4).
That moment is Julian Day 2461335.5417. The previous new moon was on October 10 at 11:50 a.m. EDT, so the moon is 11.38 days old — 38.5% of the way through a mean cycle.
The simple cosine estimate gives 87.6% lit, but this lunation is a slow one (the full moon is not until 12:12 a.m. on October 26), and the phase-angle method gives the correct 80.8%. Because no principal phase falls on October 21, the day is a waxing gibbous.
The phases at a glance
| Phase | Moon age (approx.) | Lit fraction | Visible |
|---|---|---|---|
| New Moon | 0 days | 0% | Not visible; rises and sets with the Sun |
| Waxing Crescent | 1–6 days | 1–49% | Western sky after sunset |
| First Quarter | about 7.4 days | 50% | High at sunset, sets near midnight |
| Waxing Gibbous | 8–14 days | 51–99% | Evening into the early morning |
| Full Moon | about 14.8 days | 100% | Rises at sunset, up all night |
| Waning Gibbous | 15–21 days | 99–51% | Rises mid-evening |
| Last Quarter | about 22.1 days | 50% | Rises near midnight, high at sunrise |
| Waning Crescent | 23–29 days | 49–1% | Eastern sky before dawn |
The ages vary by up to about 15 hours either way, which is why the table shows ranges.
Full moon names
North American folk names follow the month: Wolf (January), Snow, Worm, Pink, Flower, Strawberry, Buck, Sturgeon, Corn, Hunter’s, Beaver and Cold (December). The Harvest Moon is the full moon nearest the September equinox, so it can fall in late September or early October. A second full moon inside one calendar month is listed as a Blue Moon.
Tips for planning around the moon
For stargazing and meteor showers, aim for the days around new moon, when the sky is darkest. For night photography of the moon itself, the days just before full give the most surface detail along the terminator. Pair the phase with the sunrise and sunset calculator to see how dark it will be, or print a month with holidays from the printable calendar. Easter’s date is also set by the moon — the Easter date calculator shows how.
Frequently asked questions
How accurate is this moon phase calculator?
New, first-quarter, full and last-quarter times come from Jean Meeus's lunar phase algorithm, which agrees with almanac times to within a minute or two for dates between 1900 and 2100. The phase named for a date is therefore correct to the day; only an event within a minute or two of midnight could land on the neighboring date.
Why is the moon not exactly half lit at first quarter?
It is very close: at the instant of first or last quarter the lit fraction is about 50%. Small differences come from the moon's position slightly above or below the Sun's path, and from rounding to one decimal place.
What is a blue moon?
In popular use, a blue moon is the second full moon in a single calendar month. It happens about every two and a half years because a lunation (29.53 days) is shorter than most months. May 2026 has one: full moons on May 1 and May 31.
Does the moon look different in the Southern Hemisphere?
Yes. The phase and lit percentage are the same everywhere, but observers south of the equator see the moon upside down compared with northern viewers, so a waxing moon is lit on the left instead of the right. Use the hemisphere switch to flip the drawing.
Why do I need a time zone for a moon phase?
Phases happen at one instant worldwide, but the calendar date of that instant depends on where you are. The October 2026 full moon, for example, is late on October 25 in California but early on October 26 in New York.