Moon Phases Explained: The Lunar Cycle, Full Moon Names and Blue Moons

The Moon runs through its phases in an average of 29.53 days, from new moon to full moon and back to new. Astronomers call that cycle the synodic month. It is the reason a full moon falls about a day earlier on the calendar each month and why the same full moon date almost never repeats in consecutive years.

The table below this guide lists this year’s and next year’s full moons in US Eastern Time with their traditional names. The sections here explain what the phases are, where the 29.53 days come from, how the exact times are calculated, and what calendar terms such as Wolf Moon, blue moon and supermoon mean.

The eight phases of the Moon

Half of the Moon is always lit by the Sun. What changes is how much of that lit half faces Earth. The cycle is usually divided into eight phases:

1. New moon. The Moon sits roughly between Earth and the Sun, and the side facing us is dark.
2. Waxing crescent. A thin sliver appears in the western sky after sunset and grows each evening.
3. First quarter. The right half of the disk is lit, as seen from the United States. The Moon is a quarter of the way around its orbit.
4. Waxing gibbous. More than half is lit, and the bright part keeps growing.
5. Full moon. The Moon is on the far side of Earth from the Sun, and the whole visible face is lit.
6. Waning gibbous. The light starts shrinking from the right side.
7. Last quarter, also called third quarter. The left half is lit, and the Moon rises around midnight.
8. Waning crescent. A thin sliver rises before dawn and fades into the next new moon.

Waxing means growing and waning means shrinking. Crescent describes less than half lit; gibbous describes more than half.

Only four of these are instants you can put on a calendar. New moon, first quarter, full moon and last quarter happen when the Moon is 0, 90, 180 and 270 degrees east of the Sun along the ecliptic, as measured from Earth’s center. The crescent and gibbous phases are the stretches in between. That is why calendars, including the moon symbols on each 2026 calendar page, show four phases per cycle rather than eight.

Why the lunar cycle lasts 29.53 days

The Moon takes 27.32 days to circle Earth once relative to the stars. That is the sidereal month. The phase cycle is longer because Earth is moving around the Sun at the same time, so the Moon has to travel a bit farther to line up with the Sun again.

The arithmetic is short:

1. The Moon moves 360 ÷ 27.32166 = 13.18 degrees per day against the stars.
2. The Sun appears to move 360 ÷ 365.25 = 0.99 degrees per day in the same direction.
3. The Moon gains on the Sun by 13.18 − 0.99 = 12.19 degrees per day.
4. Gaining a full 360 degrees takes 360 ÷ 12.19 = 29.53 days.

NASA’s figure for the mean synodic month is 29.53059 days, or 29 days, 12 hours and 44 minutes. Real cycles drift around that average by up to about seven hours, because the Moon’s orbit is an ellipse and the Sun’s gravity keeps tugging on it.

The same 12.19 degrees explains why the Moon rises later each day. Earth turns 15 degrees per hour relative to the Sun, so making up 12.19 degrees takes about 49 minutes. On average, moonrise comes about 49 minutes later each day, although the actual delay varies with the season and your latitude.

How moon phase dates are calculated

Modern phase tables do not come from observation. They come from formulas. NASA’s long-range phase catalog by Fred Espenak and the calculations behind Old Calendars both use the methods in Jean Meeus’s Astronomical Algorithms. The process has four steps:

1. Count whole cycles from a reference new moon and multiply by the mean synodic month to get an approximate time.
2. Add correction terms for the shape of the Moon’s orbit, the Sun’s pull and other effects. The largest correction terms are worth several hours each.
3. Convert from the uniform time scale used in orbital calculations to Universal Time, which follows Earth’s slightly irregular rotation.
4. Convert Universal Time to a local time zone. Old Calendars uses US Eastern Time.

Worked example: the 13 full moons of 2026

NASA’s table puts the first full moon of 2026 on January 3 at 10:03 Universal Time. Twelve average cycles later:

• 12 × 29.53059 = 354.367 days, which is 354 days, 8 hours and 49 minutes.
• January 3 is day 3 of the year. Day 3 + 354 = day 357, which is December 23 in a common year.
• Adding 8 hours and 49 minutes to 10:03 gives a predicted full moon at 18:52 UT on December 23.

The real full moon in NASA’s table comes at 01:28 UT on December 24, about six and a half hours later than the simple average predicts. That gap is the correction step at work, and it is why serious tables never just add 29.53 days.

The example also shows why 2026 has 13 full moons. Twelve cycles use only about 354 days, roughly 11 days short of a calendar year. When the first full moon falls early in January, a 13th one fits before December 31.

Then step 4 matters. The December full moon at 01:28 UT on December 24 is 8:28 p.m. Eastern Time on December 23. Americans and Europeans can correctly print different dates for the same event. Our guide to time zones in the United States covers more of these date shifts.

Full moon names used in the United States

American calendars and almanacs give each month’s full moon a traditional name. The Farmers’ Almanac describes them as a mix of Indigenous North American traditions, especially those of Algonquian-speaking peoples in the Northeast, plus colonial and European customs. The names varied from nation to nation, so no single list is “official.” The common set runs:

• January: Wolf Moon
• February: Snow Moon
• March: Worm Moon
• April: Pink Moon, named for early spring wildflowers such as creeping phlox
• May: Flower Moon
• June: Strawberry Moon
• July: Buck Moon
• August: Sturgeon Moon, for the fish once caught in the Great Lakes
• September: Corn Moon, or Harvest Moon
• October: Hunter’s Moon
• November: Beaver Moon
• December: Cold Moon

The Harvest Moon is the one name tied to astronomy rather than to the month. It is the full moon closest to the September equinox. In 2026 that was September 26. In 2025 it was October 6, and in 2028 it will be October 3, so in those years the September full moon keeps the name Corn Moon. The Hunter’s Moon is simply the next full moon after the Harvest Moon, which was October 26 in 2026. The equinoxes and solstices guide lists the equinox dates the rule depends on.

Blue moons: two competing definitions

A blue moon has nothing to do with color. It is an extra full moon, and two rules are in use.

Calendar blue moon. The second full moon in a single calendar month. May 2026 had one: full moons on May 1 and May 31. This only works because a lunar cycle, at 29.53 days, is shorter than every month except February. The next calendar blue moon in Eastern Time falls on December 31, 2028, the second December full moon of that year.

Seasonal blue moon. The older rule comes from the Maine Farmers’ Almanac. An astronomical season normally holds three full moons. When it holds four, the third is the blue moon. The summer of 2024 had four, so the full moon of August 19, 2024 was a seasonal blue moon. Spring 2027 will have four as well, making May 20, 2027 a seasonal blue moon.

The calendar definition spread after a 1946 Sky & Telescope article misread the almanac’s seasonal rule. It has since become the meaning most people know.

Both kinds happen about seven times every 19 years. The numbers explain why: 19 years × 365.2422 days = 6,939.6 days, and 6,939.6 ÷ 29.53059 = 235.0 lunations. Nineteen years hold 228 calendar months, so 235 − 228 = 7 full moons have to double up somewhere.

Supermoons, micromoons and eclipses

The Moon’s distance from Earth changes every orbit. NASA’s figures put the closest point, perigee, at an average of about 363,400 km and the farthest point, apogee, at about 405,500 km. A full moon near perigee is popularly called a supermoon, and one near apogee a micromoon.

The word supermoon came from astrology and was later adopted by some astronomers and the press, so definitions vary. The Royal Observatory Greenwich counts a full moon within 10 percent of the closest distance. Compared with a full moon at apogee, a supermoon can look up to 14 percent bigger and 30 percent brighter. By that rule, the Royal Observatory lists the full moons of December 24, 2026 and January 22, 2027 as supermoons. The first of those is the evening of December 23 in US time zones.

Phases also decide when eclipses can happen. A lunar eclipse can occur only at full moon and a solar eclipse only at new moon, when the Sun, Earth and Moon line up closely. Most months they miss, because the Moon’s orbit is tilted about 5 degrees to the plane of Earth’s orbit around the Sun. In 2026, NASA’s table flags a total lunar eclipse at the March 3 full moon and a partial one at the August 28 full moon.

Reading moon phases on a calendar page

Each year calendar on this site marks new moon, first quarter, full moon and last quarter. Keep three things in mind when you use them for research:

• Times are Eastern Time. A phase shortly after midnight Eastern can belong to the previous date on the West Coast or in Hawaii.
• Dates shift by about 11 days a year. Twelve lunations fall about 11 days short of a calendar year, so a full moon on a given date this year will usually fall around 11 days earlier next year. Compare the 2027 calendar with 2026 to see the slide.
• Religious calendars use their own rules. Easter depends on an ecclesiastical full moon from tables rather than the astronomical one, as explained in how Easter’s date is calculated. Lunar New Year begins on a new moon, and Islamic months such as Ramadan begin with the crescent, which many communities confirm by sighting.