The Four Kinds of Lunar Month: Why the Moon Has Four Different Months

Ask how long a lunar month is and you will usually hear 29.5 days. That is correct for the month that runs the Hijri calendar. But the Moon also goes round the Earth in 27.3 days, and astronomers have two more "months" of 27.2 and 27.6 days. They are all real, they are all measured against something different, and each one governs a different thing you can see in the sky.

The short version:

• Sidereal month, 27.32 days: the Moon returns to the same place among the stars.
• Draconic month, 27.21 days: the Moon returns to the same node, the point where its orbit crosses the Sun's path. This one decides eclipses.
• Anomalistic month, 27.55 days: the Moon returns to perigee, its closest point to Earth. This one decides the supermoon cycle.
• Synodic month, 29.53 days: the Moon returns to the same phase, from new moon to new moon. This is the month of the Hijri calendar.

Every figure in this article was recomputed with the astronomy-engine library that powers the Moon dashboard and the calendar.

Why One Orbit Gives Several Months

A month is the time between two repeats of something. The trouble is that the Moon's orbit is not a fixed ring. Three things keep moving while the Moon goes round:

1. The Earth moves around the Sun. To see the same phase again, the Moon has to catch up with the Sun, which has drifted about 27 degrees along the sky in the meantime.
2. The orbit's tilt turns. The Moon's orbit is tipped about 5 degrees to the Sun's path, and the two crossing points, the nodes, slowly slide backwards.
3. The orbit's long axis turns. The ellipse itself rotates in its own plane, so the point of closest approach drifts forwards.

Pick a different "same place" and you get a different month. The Moon's path across the sky, which we describe in why the Moon's path changes so fast, is governed by the first of these.

The Four Months in One Table

The engine confirms the first three. Averaged over 2000 to 2030, the Moon's J2000 ecliptic longitude gives 27.32 days for the sidereal month, 402 consecutive ascending-node crossings give 27.21 days, and 397 consecutive perigees give 27.55 days. For the synodic month, all 2,473 intervals between new moons from 1900 to 2100 average 29.53 days.

The Four Months to Scale

The three orbit-based months differ by less than half a day, which is why a bar chart that starts at zero makes them look almost the same. The synodic month is the odd one out, about two days longer.

The lower panel zooms in on the synodic month itself. Each dot is one real lunation between 2026 and 2030, computed from the engine's new-moon times. We come back to why they are scattered in the last section.

The Sidereal Month: 27.32 Days

If you watch the Moon pass a bright star, and then wait for it to pass the same star again, you will wait about 27.32 days. This is the true time the Moon takes to go once round the Earth, measured against the distant stars.

It is the month behind the cycle of high and low arcs in the monthly arc article, which rounds it to 27.3 days.

The Synodic Month: 29.53 Days

Now watch the Sun as well. While the Moon goes once round the stars in 27.32 days, the Earth has carried us along its own orbit, so the Sun appears to have moved forward by about 27 degrees. The Moon has to travel those extra degrees to line up with the Sun again, which costs about 2.2 days.

The arithmetic is exact. Combine the Moon's 27.3217-day circuit with the Sun's 365.2564-day return to the same star position, and the Moon catches the Sun again after 1 divided by (1/27.3217 minus 1/365.2564), which is 29.5306 days. The engine's mean for new-moon-to-new-moon over 2000 to 2030 is 29.529 days, and over 1900 to 2100 it is 29.530.

This is the month that matters for a calendar, because the phases are what people see. A Hijri month starts with the new crescent, and the crescent is visible only after the conjunction, as explained in how the Islamic lunar calendar works. Since a calendar month must have a whole number of days, Hijri months are 29 or 30 days long, with the mean landing between them at 29.53. Twelve of them make 354.37 days, which is 10.88 days short of the 365.24-day tropical year. That gap is why Ramadan moves earlier through the seasons.

The three engines in the triple-engine Hijri calendar are three ways to turn this rhythm into whole days. The tabular engine uses the mean directly, in a 30-year cycle of 10,631 days, which is 29.53056 days per month. The astronomical engine uses the actual moment of conjunction. The Umm al-Qura calendar applies a fixed astronomical rule. None of them uses the 27-day months.

The Draconic Month: 27.21 Days

The Moon's orbit is tilted about 5 degrees to the Sun's path, so the Moon is usually a little north or south of the line the Sun follows. Twice a month it crosses that line, at the two nodes. The draconic month (from the Latin draco, the dragon of old eclipse stories) is the time between two crossings at the same node.

It is shorter than the sidereal month, 27.21 days against 27.32, because the nodes slide westwards along the Sun's path, completing one lap in about 18.6 years. The Moon meets the node slightly early each time. The engine gives 27.212 days as the mean of 402 ascending-node crossings from 2000 to 2030, and 18.60 years follows from the difference between the two months.

An eclipse needs the Sun and Moon to line up in longitude (a new or full moon) while the Moon is near a node. The Sun meets the same node every 346.62 days, the eclipse year, which is the synodic month times the draconic month divided by their difference. That figure, and why it forces solar and lunar eclipses into pairs, is the subject of why eclipses come in pairs.

The Anomalistic Month: 27.55 Days

The Moon's orbit is an ellipse. At its closest, the perigee, the Moon is between about 356,500 and 370,300 km away, and at its farthest, the apogee, between about 404,100 and 406,700 km (engine values for every perigee and apogee from 2000 to 2030). The anomalistic month is the time from one perigee to the next.

It is longer than the sidereal month, 27.55 days against 27.32, because the ellipse itself turns forward in the same direction as the Moon, once every 8.85 years. After one lap the Moon still has a little further to go to reach the new perigee.

An individual interval is far less regular than the mean suggests. Across 397 perigees from 2000 to 2030 the gap between two perigees ranges from 24.66 to 28.55 days. Quoting 27.55 is accurate on average, and in any single case it can be off by days.

Supermoons and the 411.8-day cycle

A full moon happens every synodic month, and a perigee every anomalistic month. They fall together again after the beat period of the two, 27.5546 times 29.5306 divided by their difference, or 411.8 days (about 13.9 lunations). Full moons that sit closest to Earth therefore recur roughly every 14 lunations. In the engine's list for 2026 to 2030, the closest full moons fall on 24 December 2026, 10 February 2028, 30 March 2029 and 17 May 2030, with gaps of 413, 414 and 413 days.

This is the cycle behind the popular supermoon headlines. The anomalistic month is the one that counts here.

How the Months Connect: A Few Useful Coincidences

The months are different numbers, but they fit together in surprising ways:

The last three rows are the Saros. After 223 synodic months, the Sun, Moon and node line up almost identically, and the Moon is at nearly the same distance (239 anomalistic months come within about 0.2 day). That is why an eclipse is followed a little over 18 years later by a similar one, shifted westward on Earth by about 8 hours of the planet's rotation (the extra 0.32 day).

Why Even the Synodic Month Is Not Constant

The 29.53-day figure is a mean. Nobody has ever measured a lunar month of exactly that length. Taking every gap between consecutive new moons from 1900 to 2100, the engine gives:

Restricted to the new moons of 2026 to 2030, the range is 29.28 to 29.81 days, with the shortest beginning on 15 June 2026 and the longest on 9 December 2026.

Two things cause the variation, and both can be seen in the engine's numbers.

The Moon's elliptical orbit. The Moon moves fastest near perigee and slowest near apogee. Because the Moon is chasing the Sun, the length of a month depends on where in its orbit the Moon starts. For the new moons from 1990 to 2040 the correlation between a lunation's length and the Moon's distance at the new moon that begins it is 0.87: a month that starts with the Moon far away tends to be long, and one that starts with the Moon close tends to be short.

The Earth's elliptical orbit. The Sun appears to move fastest along the sky when the Earth is nearest to it, in early January, so the Moon has farther to go to catch up. In the same 1990 to 2040 run, lunations beginning in January average 29.607 days and those beginning in July average 29.441 days.

This matters for the Hijri calendar. Conjunction times wander by hours around the mean schedule. When a conjunction falls close to a day boundary or to sunset, that can decide whether a month has 29 or 30 days, which is part of why communities disagree, as in why Muslims start Ramadan on different days explains.

Which Month Is Which: A Quick Guide

• Planning a sighting or checking the Hijri date: synodic. Use the calendar, and on the Moon dashboard see the Moon's age and phase.
• Predicting eclipses: draconic, combined with the synodic. Use the eclipse tool. One example is the total eclipse of 2 August 2027, covered in total solar eclipse over Mecca.
• Moon distance and supermoons: anomalistic. Moon through the stars: sidereal.

A solar eclipse is a new moon at a node, so it falls at the end of the Hijri month, most often on the 29th, a pattern we measure in why solar eclipses fall on the 29th.

Frequently Asked Questions

How long is a lunar month?

It depends which one you mean. The month of the phases and the Hijri calendar, the synodic month, averages 29.53 days. The Moon takes 27.32 days to go once round the stars. A particular synodic month can be anywhere from about 29.27 to 29.83 days.

Why is the synodic month longer than the sidereal month?

Because the Earth moves around the Sun while the Moon orbits. After one lap against the stars, the Sun has appeared to move forward about 27 degrees, and the Moon needs roughly 2.2 more days to catch up and show the same phase.

Why are Hijri months 29 or 30 days and never 29.53?

A calendar day is a whole day, so a month is either 29 or 30 days. The mean is 29.53, and over several months the 29s and 30s average out to it. Which one a given month gets depends on the rule: the moment of conjunction, a moonset condition, a fixed table, or actual sighting.

Which month decides when eclipses happen?

The draconic month, 27.21 days, tied to the Moon's crossings of the Sun's path, together with the synodic month. An eclipse needs a new or full moon within a few degrees of a node. The Sun returns to the same node every 346.62 days.

Is the 29.53-day month always the same length?

No. Individual lunations range from about 29.27 to 29.83 days, a spread of roughly 13 hours. The causes are the elliptical orbits of both the Moon and the Earth.

Conclusion

The Moon has one orbit and four months, because each month counts the return of something different: the stars, the node, the perigee or the Sun. For the calendar, only the synodic month matters, and even that varies by hours from one lunation to the next. For eclipses and supermoons, the quieter 27-day months do the work.

Open the Moon dashboard to see today's phase and distance, and the eclipse tool to see where the draconic month is taking the next shadow.

See Also

• The Moon's 18.6-Year Cycle, and Why Northern Autumn Crescents Hug the Horizon Until the Early 2030s
• The day the Moon does not rise: why moonrise skips a day every month

References

• Meeus, J. (1998). Astronomical Algorithms (2nd ed.). Willmann-Bell. Chapter 49, phases of the Moon, and Chapter 50, perigee and apogee of the Moon.
• Espenak, F. (NASA). Eclipses and the Saros. https://eclipse.gsfc.nasa.gov/
• Astronomy Engine documentation: SearchMoonPhase, SearchMoonNode, SearchLunarApsis. https://github.com/cosinekitty/astronomy