Lunar Month Lengths: Why the Moon Returns to a Star in 27.3 Days but Repeats a Phase in 29.5

The Moon does not have one month. Ask how long it takes to go around Earth and you can get two honest answers, 27.3 days and 29.5 days. Both are correct. They answer different questions, and mixing them up is one of the most common slips in lunar timekeeping.

This guide separates the two lengths, lists the official numbers and explains where the extra two days come from. It ends with a short task in the Clock Dial.

Two ways to count one lap

The first count follows the stars. Imagine a bright star sitting right next to the Moon tonight. The sidereal month is the time the Moon needs to come back to that same spot against the star background. NASA's fact sheet notes describe it as the time to orbit once relative to the fixed background stars. The U.S. Naval Observatory defines it as the mean period of revolution with respect to the background stars.

The second count follows the Sun. The synodic month is the time from one New Moon to the next, which is also the time from one phase to the same phase again. NASA's notes put it as the time from full Moon to full Moon. The Naval Observatory defines it as the period between successive new moons. NASA's eclipse pages explain why this is the familiar one: the Moon shines by reflected sunlight, so its geometry relative to the Sun and Earth sets its phase.

A small label warning helps here. NASA's Moon Fact Sheet lists the sidereal value in a row named "Revolution period." The Planetary Fact Sheet Notes explain that for the Moon, the orbital period given is the sidereal one, and that the full-Moon-to-full-Moon period is 29.53 days.

The numbers side by side

Every official page agrees. They differ only in how many decimals they print.

Two other months appear in the figure. NASA's eclipse pages define the anomalistic month as the Moon's trip from its closest point to Earth (perigee) back to perigee, with a mean of 27.55455 days. They define the draconic month as one revolution measured against the point where the Moon's path crosses the plane of Earth's orbit going north, with a mean of 27.21222 days. NASA calls these other months crucial to predicting eclipses. This guide stays with the first two.

One real disagreement is worth naming. NOAA's solar calculator glossary defines a month as the time for the Moon to complete one orbit of Earth and then gives 29.53059 days. NASA's pages keep the two ideas apart: one orbit against the stars is 27.32 days, and 29.53 days is the cycle of phases. The number in that entry matches the phase cycle. The wording about one orbit is where the two ideas blur.

Why the phase cycle takes longer

The Moon is not circling a stationary Earth. NASA's eclipse pages explain that as the Moon revolves around Earth, both objects also move along their orbit around the Sun. After one revolution with respect to the stars, the Moon must go a little farther to catch up to the same position relative to the Sun and Earth. NASA states that the synodic month is nearly 2.21 days longer than the sidereal month. NASA's StarChild page says the extra time is just over 2 days.

The drawing shows the Moon at an age of 27.32 days. By then it has circled Earth once against the stars, but it is still a little short of the New Moon position. It needs about two more days to line up with the Sun again.

You can check the size of the gap with simple division of sourced numbers. The Naval Observatory puts the motion caused by Earth's orbit at about 1 degree per day. A month of about 29.5 days therefore carries Earth roughly 30 degrees, and NASA uses the same figure of about 30 degrees. The Moon covers 360 degrees in 27.32 days, which is about 13.2 degrees per day. Catching up 30 degrees at that speed takes about 2.3 days. That is close to NASA's 2.21 days, and the small difference comes from rounding the 30 degrees.

The same idea sits behind sidereal and solar days. Earth turns once against the stars in a little under 24 hours, but it needs a bit more turning to bring the Sun back. For the Moon, the extra motion is two days instead of four minutes.

Even the long month is an average

The 29.53 days is a mean. NASA says the duration of a lunation, meaning New Moon to New Moon, varies from its mean value by up to seven hours. Over the 5,000-year span covered in NASA's eclipse catalog, the shortest lunation lasted 29.26574 days and the longest lasted 29.84089 days.

NASA links the swing to the Moon's distance. A lunation runs shorter than average when New Moon happens near perigee and longer when it happens near apogee, the farthest point. NASA adds that Earth's own elliptical orbit also factors into the length of a lunation.

The Moon Dial counts a Moon's age on the 29.5-day clock, and the New Moon guide covers the phase where that count begins. The Naval Observatory calls the days counted from New Moon the Moon's age, and it notes that the cycle of phases is shorter than most calendar months. That is why a month on the calendar and a month on the Moon drift apart. For what that does to lunar calendars, see the lunar calendar guide.

Common mistakes

• Saying the Moon orbits Earth in 29.5 days. One orbit against the stars takes about 27.3 days. The 29.5 days is the cycle of phases.
• Using one number for every purpose. Use 27.32 days for a return to the same star and 29.53 days for a return to the same phase.
• Mixing up sidereal, anomalistic and draconic months. The values 27.32, 27.55 and 27.21 days look alike but measure three different returns.
• Treating 29.53 days as exact for every lunation. Real lunations can differ from the mean by up to seven hours, according to NASA.
• Blaming the stars for the phases. The phases come from the Moon's position relative to the Sun and Earth. The stars only mark where the Moon is in the sky.
• Treating a calendar month as one lunation. The cycle of phases is shorter than most calendar months, says the Naval Observatory.

Try it in the instrument

Open the Clock Dial and set the sky to the sidereal month, the Moon's return to the same star. The target is about 27.32 days. NASA's Moon Fact Sheet lists 27.3217 days, and NASA's eclipse pages list 27.32166 days.

The relation behind the answer is a subtraction. Start with the phase cycle of 29.53 days, the one the Moon Dial uses. Take away the catch-up time of nearly 2.21 days that NASA gives, and you land on 27.32 days. Going the other way, add about 2.2 days to the sidereal month and you get the phase cycle. The dial is a simplified model with a window of accepted answers, so the idea matters more than the last decimal. Just keep 29.5 out of this answer, because that is the other month.

Questions people ask

How long is the sidereal month?

About 27.32 days. NASA's Moon Fact Sheet gives 27.3217 days, and the Naval Observatory gives approximately 27.322 days. It is the time for the Moon to return to the same position against the stars.

How long is the synodic month?

About 29.53 days. NASA says it is the time from full Moon to full Moon, and the Naval Observatory defines it as the time between successive new moons. It is also called a lunation.

Why is the synodic month longer than the sidereal month?

Earth moves around the Sun while the Moon moves around Earth. After one lap against the stars, the Moon must travel a little farther to line up with the Sun again. NASA puts the difference at nearly 2.21 days.

Does every lunation last exactly 29.53 days?

No. NASA says a lunation can differ from the mean by up to seven hours. Over 5,000 years of eclipse data, lunations ranged from 29.26574 to 29.84089 days.

Which month do lunar calendars use?

Lunar calendars are built on the phases, so they follow the synodic month. NASA notes that a solar year is not a whole number of lunations, so most lunar calendars are lunisolar and add extra months to keep the seasons in step.

Sources

• Moon Fact Sheet, NASA Space Science Data Coordinated Archive. https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html. Opened November 9, 2026.
• Planetary Fact Sheet Notes, NASA Space Science Data Coordinated Archive. https://nssdc.gsfc.nasa.gov/planetary/factsheet/planetfact_notes.html. Opened November 9, 2026.
• NASA - Eclipses and the Moon's Orbit, NASA Goddard Space Flight Center (Fred Espenak). https://eclipse.gsfc.nasa.gov/SEhelp/moonorbit.html. Opened November 10, 2026.
• StarChild Question of the Month for April 2001, NASA StarChild, Goddard Space Flight Center. https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question32.html. Opened November 10, 2026.
• Glossary, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/asa_glossary. Opened November 10, 2026.
• Phases of the Moon and Percent of the Moon Illuminated, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/moon_phases. Opened November 10, 2026.
• The Equation of Time, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/eqtime. Opened November 9, 2026.
• Tides and Currents Glossary, NOAA Tides & Currents. https://tidesandcurrents.noaa.gov/glossary.html. Opened November 10, 2026.
• Solar Calculator Glossary, NOAA Global Monitoring Laboratory. https://gml.noaa.gov/grad/solcalc/glossary.html. Opened November 10, 2026.

We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.

Read next

• Lunar CalendarsA lunar calendar counts months from one cycle of Moon phases to the next.
• The Sidereal Day and the Solar DayA sidereal day is one turn of Earth measured against the stars, about 23 hours 56 minutes 4 seconds.