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The Moon’s month

Why the Moon’s phases outlast its orbit

The Moon circles Earth against the stars in 27.3 days, yet its phases take 29.5 days to repeat. Follow one month to see where the extra two days go.

01 · Two references

Time the Moon against a star, then against the Sun.

The Moon circles Earth while Earth travels around the Sun. Pick a distant star that lies directly behind the Sun at new Moon. The star is so far away that its direction from Earth never changes. The Sun’s direction does, because Earth keeps moving.

The Moon's orbit against a distant star and against the Sun Seen from north of Earth's orbit. Earth moves along its orbit around the Sun while the Moon circles Earth. One line from Earth points to a distant star, always in the same direction; another points to the Sun. At new Moon both lines and the Moon line up. Back in front of the same stars. The Sun has moved 27° on, so the phase is not yet new. Sun Earth at new Moon Earth Moon Dashed line toward a distant star Gold line toward the Sun
Day 27.3Back in front of the same stars. The Sun has moved 27° on, so the phase is not yet new.
Seen from north of Earth’s orbit, with both orbits drawn as circles at their average speeds. Sizes and distances are not to scale, but the angles are: Earth’s 27° step along its orbit is drawn as 27°.

After one circuit against the star, about 27.3 days, the Sun has shifted about 27° east among the stars. The Moon needs about two more days to reach it again, and only then is the phase new.

02 · A real month

The gap is there in every month.

Real orbits are not circles, so the exact lengths change from month to month. The ranges are narrow, though, and they never come close to meeting.

One month from the new Moon of 9 November 2026 From the new Moon of 9 November 2026, the Moon was back in front of the same stars after 27.26 days, on 6 December, and reached the next new Moon after 29.74 days, on 9 December. The 2.5 days between them were spent catching up with the Sun. Below, over 2026 to 2036, months against the stars range from 27.22 to 27.47 days and phase cycles from 29.28 to 29.82 days, so the two ranges never overlap. FROM THE NEW MOON OF 9 NOVEMBER 2026 · DAYS Back to the same stars 6 Dec · 27.26 days Next new Moon 9 Dec · 29.74 days 051015202530 Shaded: 2.5 days catching the Sun LENGTHS FROM 2026 TO 2036 · ZOOMED TO 27–30 DAYS Against the stars 27.22–27.47 Phase cycle 29.28–29.82 27282930
Calculated with the same ephemeris as the site’s Panchang. Lengths run from each event to the next of the same kind.

03 · Two Panchang counts

Tithi follows the phases; nakshatra follows the stars.

A Panchang counts the Moon both ways. Tithis divide the phase cycle into 30 parts. Nakshatras divide the circuit of the stars into 27.

Thirty tithis and twenty-seven nakshatras Two rows on one axis of days. Thirty tithis fill one phase cycle of about 29.5 days, the first fifteen in the waxing half and the rest in the waning half. Twenty-seven nakshatras fill one circuit of the stars of about 27.3 days. Each averages close to a day, but the rows drift apart, so their boundaries fall at different moments. 30 tithis · one phase cycle Light: Shukla · dark: Krishna 27 nakshatras · one circuit of the stars 051015202530 Days from a new Moon · average lengths
Both rows start together here to compare their lengths. Real tithis and nakshatras vary in length and rarely begin at the same moment.

Two ways to time one orbit

The Moon travels eastward around Earth. Measured against the distant stars, it returns to the same place in about 27.3 days. This is the sidereal month. Measured against the Sun, from one new Moon to the next and through every phase between, it takes about 29.5 days. This is the synodic month, and it is the month a lunar calendar follows.

Both describe the same motion. They differ because the second reference, the Sun, does not stay still among the stars.

Phases follow the Sun

The Sun always lights half of the Moon. The phase seen from Earth depends on how much of that lit half faces Earth, and that depends on the angle between the Moon and the Sun as seen from Earth. At new Moon the two share the same longitude along the ecliptic, and the lit half faces away. At full Moon they are 180° apart, and the lit half faces Earth.

Earth’s shadow plays no part in the ordinary phases. It falls on the Moon only during a lunar eclipse, which needs a full Moon close to one of the Moon’s nodes, explained in Rahu and Ketu. The tithi and Moon phase guide compares the visible phase with the Panchang’s count.

Where the extra two days come from

While the Moon makes one circuit against the stars, Earth moves about 27° along its orbit. Seen from Earth, the Sun’s place among the stars shifts by the same 27° eastward. So when the Moon is back in front of the same stars, the Sun is no longer there, and the Moon has to travel on for about two more days to reach it.

The same result follows from the two speeds. On average the Moon moves about 13.2° a day against the stars and the Sun about 1°, so the Moon gains on the Sun by about 12.2° a day. Gaining a full 360° takes 360 ÷ 12.2, or about 29.5 days. The hands of a clock behave the same way: the minute hand needs a little more than an hour to pass the hour hand again, because the hour hand has moved on.

One month in 2026

The new Moon of 9 November 2026 shows the gap in a real month. Times are in IST.

Moment Date and time (IST) Days after the new Moon
New Moon 9 November 2026, 12:32 0
Moon back in front of the same stars 6 December 2026, 18:49 27.26
Next new Moon 9 December 2026, 06:21 29.74

By the time the Moon was back among the same stars, the Sun had moved 27.5°. Over the whole month to the next new Moon it moved about 30°.

Neither month is fixed

The figures of 27.3 and 29.5 days are averages. Between 2026 and 2036, months measured against the stars run from 27.22 to 27.47 days, and phase cycles from 29.28 to 29.82 days.

Two things cause the variation. The Moon’s orbit is an ellipse, and the Moon moves faster when it is closer to Earth. Earth’s orbit is also slightly elliptical: in early January, when Earth is nearest the Sun, the Sun moves about 1.02° a day against the stars, against about 0.95° in early July. When the Sun moves faster, the Moon takes longer to catch it. The month in the example above, running into December, is longer than average partly for this reason.

Two counts in the Panchang

A Panchang uses both months.

Both average close to a day: a tithi about 23.6 hours and a nakshatra about 24.3 hours. Because they follow different months, their boundaries drift apart, and a tithi and a nakshatra rarely change at the same moment.

See today’s tithi and nakshatra on the Delhi Panchang, or explore another topic in Learn.