Sidereal vs Synodic Month: Why the Moon Takes 27.3 Days

Measured against the stars, the Moon goes once round the Earth in 27.32 days on average. That is the sidereal month, and it is where the familiar “27.3 days” comes from. Measured against the Sun, from one new moon to the next, the same journey takes 29.53 days: the synodic month, the month of the phases. Both are lunar months, and so are three more, each counting the Moon’s return to a different reference point.

Every individual month of all five kinds that began between January 2026 and December 2028 was measured for this page from the JPL DE421 ephemeris, to the second, and set beside the long-run averages that en-Wikipedia’s article “Lunar month” tabulates from the lunar theory of Chapront, Chapront-Touzé and Francou (2002). Single events are given as dates only.

Five Kinds of Lunar Month, 2026 to 2028

Four of the five sit within nine hours of one another, close to 27.3 days; only the synodic month is about two days longer. The separation is clean in single months too: not one sidereal, tropical or draconic month in the run lasted longer than 27.44 days, and not one synodic month was shorter than 29.28. A single month never quite equals its average, because the Moon speeds up and slows down along its elliptical orbit. The sidereal month varied by about five hours from shortest to longest; the anomalistic month, measured from one closest approach to the next, varied by 3.88 days. Over a few dozen months the averages land near the long-run figures but not on them, which is why the table keeps the two columns apart.

Why the Sidereal Month Is Shorter Than the Synodic Month

While the Moon goes round the Earth, the Earth goes round the Sun, so the Sun appears to creep eastward among the stars by about 0.9856° a day. When the Moon gets back to the star it started from, the Sun is no longer there: in a synodic month it has moved on by 29.1°. At its average speed of 13.18° a day the Moon needs another 2.21 days to cover that arc and line up with the Sun again, which is exactly the difference between the two averages, 29.53 minus 27.32.

The same fact can be written as a check on the numbers: one divided by the sidereal month, minus one divided by the synodic month, equals one divided by the year. Put in the two long-run means and the year comes out at 365.256 days. The Sun’s own sidereal returns to the start of Mesha in the site’s ingress table took 365.25 days from 2026 to 2027 and 365.26 days from 2027 to 2028. Spread over a Gregorian year, that means 13.37 sidereal months but only 12.37 synodic months, one fewer, because the Sun itself makes one circuit in that time.

The Tropical Month: About Seven Seconds Shorter

The en-Wikipedia article defines the tropical month as the return of the Moon to the same ecliptic longitude measured from the March equinox point, and explains that this point slides slowly backwards along the ecliptic through the precession of the equinoxes, so the Moon meets it a little early. The long-run means differ by 6.9 seconds. The site’s Lahiri ayanamsa formula grows at 50.30″ a year, one full circle in about 25,800 years; at the Moon’s speed, the arc it adds in one sidereal month takes 6.9 seconds to cover, which matches the tabulated difference. How that slowly growing gap moves sidereal positions away from tropical ones is set out in the Lahiri ayanamsa explainer.

The measured columns cannot show a few seconds. The 40 tropical months and the 40 sidereal months of 2026–28 begin on different days and so sample the Moon’s speed cycle differently, and their averages differ by 0.16 hours, far more than the effect being looked for. The seconds show up only in long-run averages such as the tabulated ones.

The Draconic or Nodical Month and Rahu

The Moon’s orbit is tilted to the ecliptic, so twice a month it crosses the Sun’s path: northward at the ascending node and southward at the descending node. The draconic month, also called the nodical or nodal month, runs from one passage through the same node to the next. As en-Wikipedia describes it, the line of nodes turns slowly westward, against the Moon’s motion, so the Moon reaches the node 2.63 hours sooner than it reaches the same star. Working backwards from the two long-run means, the nodes would take 6,793 days, about 18.6 years, to go once round the zodiac, the same cycle the article states.

The site’s planet table describes Rahu as the north lunar node, the point where the Moon crosses the ecliptic going north. Its ingress table moves Rahu and Ketu with a mean-node formula, a steady 0.0529° a day backwards, which comes to one circuit in about 6,805 days, close to the figure derived above. The draconic rows on this page are not that formula: each is a true crossing, the instant the Moon’s ecliptic latitude passed from south to north of zero. Single draconic months vary like the others: in 2026–28 they ran from 27.05 to 27.38 days.

The Anomalistic Month and the Perigee

The anomalistic month runs from perigee to perigee. The same article explains that the line through perigee and apogee turns in the same direction as the Moon, so the Moon must travel a little past its starting star to reach perigee again; the long-run mean is 5.59 hours longer than the sidereal month. The two means imply that the perigee goes once round in 3,233 days, about 8.85 years, matching the article’s figure.

Single anomalistic months are the most uneven of the five, from 24.67 to 28.55 days in this run. The perigees themselves differ in depth. The nearest of the span will be 356,648 km on 24 December 2026, and the anomalistic month that will open there is the longest in the table; the farthest perigee was 370,172 km on 25 February 2026. The beat between the synodic and anomalistic months, worked out from the two means, is 411.8 days: after that interval a full moon falls at the same place relative to perigee again. Which full moons count as supermoons, and how much larger they appear, is the subject of the supermoon dates page.

Which Month the Panchang Follows

A tithi is a step of 12° in the Moon’s lead over the Sun, so tithis come round again on the synodic month; how long those months and their tithis actually run is measured in how many days a lunar month has. The Moon’s rashi and nakshatra, by contrast, are positions in the sidereal zodiac, so they come round on the sidereal month. The site’s Hindi panchang month pages call this the नक्षत्र मास. All 40 sidereal months here begin at a row of the site’s Moon ingress table where the Moon enters Mesha; the next such entry after publication will be on 22 November 2026.

Anything defined by the Moon’s sign recurs on the same beat. Across the 469 pairs of successive entries into one sign in that table (2026–28), the spacing was never less than 27 d 5 h and never more than 27 d 12 h. Panchak begins when the Moon enters Kumbha, and the 17 starts in the Panchak list fall 27 d 6 h to 27 d 9 h apart. Chandrashtama, the Moon’s passage through the eighth sign from a given rashi, recurs for the same reason; its windows to the end of 2027 are listed in the chandrashtama dates. Split into twenty-seven, the sidereal month gives an average nakshatra of 24.29 hours, a little over a day; how long the Moon stays in a nakshatra measures the real spread.

Why the Purnima Nakshatra Moves Each Month

Because the two clocks run at different rates, a given tithi does not keep its nakshatra. From one full moon to the next the Moon completes a sidereal circuit and then travels another 360 × (29.53 ÷ 27.32 − 1) = 29.1°, which is 2.18 nakshatras of 13°20′. So each Purnima lands two or three nakshatras further on than the last. In the site’s table of 38 full moons (2026–28), 30 of the 37 steps were two nakshatras and 7 were three, and none was anything else. The next six full moons will fall in these nakshatras:

The nakshatra is the one holding the Moon at the instant of full moon; the nakshatra printed for a Purnima day in an almanac is read at sunrise and can differ. For the tithi and nakshatra at sunrise on a given day, see the daily panchang.

What This Page Does Not Give

What any of these months means for a person, or what it brings, is not a question this page takes up. Lengths, dates and distances can be measured and checked against an ephemeris; a meaning cannot be checked that way, so none is offered here. It also prints no clock time for any new moon, node passage or perigee, only dates and durations.

In Brief

• Sidereal month: 27.32 days (27 d 7 h 43 min), the Moon back among the same stars; the 27.3-day figure
• Synodic month: 29.53 days (29 d 12 h 44 min), new moon to new moon, 2.21 days longer because the Sun has moved on
• Tropical month: 6.9 seconds shorter than the sidereal, because the equinox point drifts
• Draconic (nodical) month: 27.21 days, node to node; the nodes circle in about 18.6 years
• Anomalistic month: 27.55 days, perigee to perigee; single months ran 24.67 to 28.55 days
• Panchang: tithis follow the synodic month, rashi and nakshatra the sidereal; each Purnima moves on by two or three nakshatras