A lunar cycle is the 29.53-day period from one new moon to the next, also called a synodic month or a lunation. Individual cycles range from about 29.27 to 29.83 days.
Key facts
- A lunar cycle (synodic month) averages 29.530589 days: 29 days 12 hours 44 minutes.
- Single cycles range from about 29.27 to 29.83 days.
- The sidereal month, the Moon's orbit relative to the stars, is 27.32 days; the synodic month is about 2.2 days longer.
- A year holds about 12.37 lunar cycles, so about 7 years in every 19 have 13 full moons.
How many days are in a lunar cycle?
A lunar cycle lasts 29.530589 days on average, which is 29 days, 12 hours and 44 minutes from one new moon to the next.
- Synodic month (new moon to new moon)
- 29.530589 days
- Shortest and longest cycles
- about 29.27 and 29.83 days
- Sidereal month (relative to the stars)
- 27.321662 days
- Anomalistic month (perigee to perigee)
- 27.554550 days
- Draconic month (node to node)
- 27.21222 days
The synodic month is the cycle that governs the phases of the moon, because the phases depend on the Moon’s angle from the Sun. The full moon falls about 14.8 days into the cycle on average, halfway between two new moons.
Single cycles vary by up to about 7 hours either side of the mean. A cycle is shorter when the new moon happens near perigee, the Moon’s closest point to Earth, where the Moon moves fastest. A cycle is longer when the new moon happens near apogee, where the Moon moves slowest.
Why is the synodic month longer than the sidereal month?
The synodic month is about 2.2 days longer than the sidereal month because Earth moves along its orbit around the Sun while the Moon orbits Earth.
The sidereal month, 27.32 days, is the time the Moon takes to return to the same position among the stars. During those 27.32 days, Earth travels about 27° of its own orbit around the Sun. The Sun’s direction, as seen from Earth, shifts by the same amount. The Moon must travel for about 2.2 more days to catch up with that new direction and line up with the Sun again.
The hands of a clock show the same effect. The minute hand laps the dial in 60 minutes, but it needs about 65 minutes to catch up with the moving hour hand. The Moon is the minute hand, and the Sun’s slowly shifting direction is the hour hand. The geometry of the angles themselves is laid out in why the moon has phases.
What are the anomalistic and draconic months?
The anomalistic month, 27.554550 days, runs from one perigee to the next, and the draconic month, 27.21222 days, runs from one crossing of Earth’s orbital plane to the next.
| Month | Measured from | Length (days) | What it explains |
|---|---|---|---|
| Synodic | New moon to new moon | 29.530589 | The cycle of phases |
| Sidereal | Same position among the stars | 27.321662 | The Moon’s true orbital period |
| Anomalistic | Perigee to perigee | 27.554550 | Changes in the Moon’s distance and size |
| Draconic | Ascending node to ascending node | 27.21222 | When eclipses are possible |
The anomalistic month tracks the Moon’s distance. A full moon that falls near perigee is a supermoon. Because the anomalistic month is about 2 days shorter than the synodic month, full moons drift in and out of step with perigee, and the closest match returns about every 14 lunar cycles, or about 412 days.
The draconic month tracks the two nodes, the points where the Moon’s orbit, tilted 5.14°, crosses Earth’s orbital plane. An eclipse can only happen when a new moon or full moon falls near a node. The interplay of the synodic and draconic months sets up eclipse seasons about every 173 days, and the lunar eclipse page lists the next ones.
When does a lunar cycle begin?
A lunar cycle begins at the instant of new moon, when the Moon and the Sun share the same ecliptic longitude and the Moon’s lit half faces fully away from Earth.
Astronomers time that instant to the minute, and it falls at the same moment everywhere on Earth. Calendars handle it in two ways:
- Calculated new moon. The month starts on the calendar day that contains the new moon instant. This site names days this way, so the local date of a new moon can differ by one day between time zones.
- First visible crescent. The month starts on the evening the first thin crescent is seen after sunset. That sighting always comes after the astronomical new moon, because the Moon must first move far enough from the Sun to be seen in twilight.
The Moon’s age, given in days, counts from the most recent new moon. An age of 7 days means a Moon near first quarter, and an age of 15 days means a Moon close to full.
How many lunar cycles are in a year?
A year contains about 12.37 lunar cycles, so most calendar years have 12 full moons, and about 7 years in every 19 have 13.
Twelve lunar cycles add up to about 354.4 days, roughly 11 days short of a 365.24-day year. Those leftover days add up to an extra full moon roughly every 2.7 years. The pattern repeats closely every 19 years, because 235 lunar cycles almost exactly equal 19 years, a relationship known as the Metonic cycle.
The extra full moon is the origin of the Blue Moon. In a year with 13 full moons, one calendar month or one astronomical season ends up with an extra full moon. The moon phase calendar shows how the four primary phases fall across each month of the year.
How do lunar calendars use the lunar cycle?
Lunar calendars count months by the lunar cycle; purely lunar calendars drift through the seasons, while lunisolar calendars add a leap month to stay in step with the solar year.
- Islamic (Hijri) calendar. This calendar is purely lunar, with 12 months that alternate between 29 and 30 days, giving a year of 354 or 355 days. Months such as Ramadan traditionally begin with the sighting of the waxing crescent after the new moon. Because the year is about 11 days shorter than the solar year, each month moves through all the seasons over about 32.5 years.
- Hebrew calendar. This calendar is lunisolar, with months of 29 or 30 days that begin at about the time of the new moon. A leap month is added in 7 years out of every 19, following the Metonic cycle, so festivals stay in their seasons.
- Chinese calendar. This calendar is also lunisolar, and each month begins on the day of the new moon. A leap month is inserted from time to time to keep the months aligned with the seasons. Chinese New Year falls on a new moon between January 21 and February 20, usually the second new moon after the winter solstice.
The English word month comes from the same root as moon. Old English monað is related to the word for moon, and a month was originally the interval from one new moon to the next. The date of the next new moon, when a new lunar cycle begins, is on its own page.