Moon Phase Calendar

Reading the Moon

The Moon makes no light of its own; we only ever see the half the Sun is lighting. As the Moon circles Earth once every 29.5 days (the synodic month), the angle between the Sun, Earth and Moon changes, so a different slice of that lit half faces us. That slow change is the cycle of phases, and the calendar above draws each day at the exact shape the Moon shows on that date. The full geometry, with a synced orbit-and-globe view, is on the Moon Phases lesson.

The four principal phases

Four moments anchor the cycle. At the new moon the Moon sits between us and the Sun and its lit side faces away, so it is invisible. About a week later the first quarter shows a half disk lit on the right. At the full moon the Moon is opposite the Sun and fully lit. Another week on, the last quarter is a half disk lit on the left. Between them the Moon is a crescent or gibbous, waxing (growing) on the way to full and waning (shrinking) on the way back to new.

Full moon names and the blue moon

Each month's full moon carries an old almanac name drawn from the seasons: the Wolf Moon of deep winter, the Strawberry Moon of June, the Harvest Moon near the autumn equinox. Because 29.5 days is a touch shorter than most calendar months, now and then a single month holds two full moons; the second is the famous blue moon, which is not really blue. Eclipses are the special case where a new or full moon lands on a lunar node, which you can chase on the Eclipse & Saros Explorer.

The 19-year Metonic cycle

Stack 235 lunar months end to end and they come out almost exactly 19 years, missing by only about two hours. The Greek astronomer Meton built a calendar on this in 432 BC, and it is still the reason a given moon phase falls on nearly the same date 19 years apart, and why the date of Easter and the Hebrew calendar follow a 19-year rhythm. The small leftover drift is mopped up by the 76-year Callippic cycle. The calculator above lets you watch any date's Moon return almost unchanged across a lifetime. You can place this cycle among the sky's other rhythms on the Cycles by Length page.

See more of tonight's sky

For where the planets are and what is visible right now, open Today in the Sky, and to find your local sunrise, sunset and twilight times, use the day-length calculator.

Frequently asked questions

What moon phase is it tonight?

The card at the top shows tonight's phase, drawn at the Moon's true illuminated shape, with the percent lit and whether it is waxing (growing) or waning (shrinking), plus the dates of the next full and new moons.

What time does the Moon rise and set today?

Pick a location (or tap Use my location) and the Moon tonight panel shows today's moonrise, moonset and the time and height of the Moon's highest point, in your device's time zone. Because the Moon rises about 50 minutes later each day, some days have only a moonrise or only a moonset within the calendar day.

What are the traditional full moon names?

Each month's full moon has an old almanac name: the Wolf Moon in January, Snow in February, Worm in March, Pink in April, Flower in May, Strawberry in June, Buck in July, Sturgeon in August, Harvest or Corn in September, Hunter's in October, Beaver in November and the Cold Moon in December.

What is a blue moon?

A blue moon is the second full moon in a single calendar month. Because the Moon's cycle is about 29.5 days, slightly shorter than most months, a month can occasionally hold two full moons; the second is called a blue moon. It is not actually blue.

What is a supermoon?

A supermoon is a full moon that falls near perigee, the Moon's closest point to Earth, so it looks a little larger and brighter than usual. There is no official cutoff, but a full moon closer than about 360,000 km is the common rule of thumb; such a moon looks roughly 7 percent wider and 15 percent brighter than an average full moon.

What is the Metonic cycle?

The Metonic cycle is a span of 235 lunar months that comes out almost exactly equal to 19 years, the two differing by only about 2 hours. Because of this near-match, the Moon returns to nearly the same phase on the same calendar date every 19 years. It is named for Meton of Athens, who described it in 432 BC, and it still underlies the Hebrew calendar and the calculation of the date of Easter.

Does the moon phase repeat every 19 years?

Yes, very nearly. Because 235 lunar months almost exactly equal 19 calendar years, the Moon shows almost the same phase on a given date 19 years later, and again after 38, 57 and 76 years. The match drifts slowly: the months run about 2 hours longer than the years, and the Moon's uneven orbital speed and the calendar's leap days add a little more, so by four cycles (76 years, the Callippic cycle) the phase has slipped about a day and calendars drop a day to stay in step.

What is the Golden Number?

The Golden Number is a year's position from 1 to 19 within the Metonic cycle, found by dividing the year by 19 and adding one to the remainder. It marks where the year sits in the 19-year rhythm of the Moon's phases and was long used, alongside other rules, to fix the date of Easter.

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