How often is there a full moon?

On average every 29.53 days — the synodic month, the time the Moon takes to return to the same position relative to the Sun. Because that is slightly shorter than most calendar months, the full moon drifts about one day earlier each month, and occasionally a month gets two.

Can there be two full moons in one month?

Yes. When a calendar month contains two full moons, the second is popularly called a blue moon. It happens roughly every two to three years — the phrase “once in a blue moon” is fair.

Is the full moon visible all night?

Essentially, yes — and it is the only phase that is. A full moon stands opposite the Sun, so it rises around sunset, culminates near midnight, and sets around sunrise.

Does the full moon have a fixed time of day?

No. The exact instant can fall at any hour — possibly while the Moon is below your horizon. You still see a virtually full moon that night.

Guides

When is the next full moon?

The date and hour below are computed the moment you open this page, with the same astronomical algorithms as the Plenilune app — in your own time zone.

The full moon is an instant, not a day

Astronomically, the full moon is the precise moment when the Moon stands opposite the Sun in the sky — when its ecliptic longitude differs from the Sun's by exactly 180°. That instant is the same for the whole planet, but the clock time is not: a full moon at 01:00 UTC on the 15th is still the evening of the 14th in New York. This is why two perfectly correct calendars can disagree by a day, and why this page shows the time in your zone.

To the eye, the Moon looks full for two or three nights in a row. If you only want to enjoy it, the evening before and the evening after the exact instant are just as good — the illuminated fraction stays above 98%.

How the date is computed

No almanac server is consulted. Your browser runs the algorithms from Jean Meeus's Astronomical Algorithms — the standard reference used by observatories and by the Plenilune app itself — which predict the instants of the lunar phases to the minute for decades around the present. The Moon's motion is one of the oldest solved problems in astronomy; it is entirely predictable, which is why a good moon app never needs a connection.

Every full moon has a name

The farmers' almanacs of North America popularised a traditional name for each month's full moon — the Wolf Moon of January, the Harvest Moon of the autumn, the Cold Moon of December. Plenilune illustrates each of the twelve with an original engraving and tells where each name comes from — and what the almanacs simplified along the way. The name of the next full moon is shown above, and each one links to its page.

Questions

How often is there a full moon?: On average every 29.53 days — the synodic month, the time the Moon takes to return to the same position relative to the Sun. Because that is slightly shorter than most calendar months, the full moon drifts about one day earlier each month, and occasionally a month gets two.
Can there be two full moons in one month?: Yes. When a calendar month contains two full moons, the second is popularly called a blue moon. It happens roughly every two to three years — the phrase “once in a blue moon” is fair.
Is the full moon visible all night?: Essentially, yes — and it is the only phase that is. A full moon stands opposite the Sun, so it rises around sunset, culminates near midnight, and sets around sunrise.
Does the full moon have a fixed time of day?: No. The exact instant can fall at any hour — possibly while the Moon is below your horizon. You still see a virtually full moon that night.

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