Moon Phase, Illumination and Tithi: Why They Are Different

Does the illuminated percentage tell you the tithi?

For a date calculation, use the tithi calculator with a place and date. A Moon icon or a photograph is useful for recognising a broad phase, but it does not contain all the information needed to determine a tithi's ending time.

Three readings that answer different questions

Illuminated fraction: what fraction of the apparent disk is lit by the Sun? “50% illuminated” concerns the face presented to the observer. It does not say that half of the entire lunar sphere has suddenly become illuminated, nor that the Moon has exactly half the brightness of a full Moon.

Phase: where is the Moon in its repeating sequence of appearances? NASA's explanation relates the changing appearance to viewing a sunlit Moon from Earth. Waxing means moving through the growing-lit part of the cycle; waning describes the diminishing-lit part.

Tithi: which 12° interval contains the directed Moon-minus-Sun longitude difference? It is an interval defined by celestial coordinates. It is not obtained by multiplying the illuminated percentage by thirty.

For example, multiplying 50% by 30 gives 15. That arithmetic would label both quarter Moons as the fifteenth tithi. The coordinate examples below instead give Shukla Ashtami and Krishna Ashtami. The percentage shortcut loses both the angular definition and the direction through the cycle.

Read the angle in the correct direction

The IMD explanatory notes define tithi by successive 12° gains of the Moon in longitude over the Sun. Take Moon longitude minus Sun longitude using the same geocentric coordinate convention. Add or subtract full 360° turns until the difference is at least 0° and below 360°. Divide by 12, discard the fractional part and add one.

Numbers 1–15 belong to Shukla Paksha; 16–30 belong to Krishna Paksha. For the number within Krishna Paksha, subtract 15. The final tithi of Shukla Paksha is Purnima; the final one of Krishna Paksha is Amavasya.

Keep the directed difference. For a Moon at 270° and Sun at 30°, it is 240°. Replacing it with the shorter 120° arc would incorrectly move the answer from the waning half to the waxing half. Another wrap example is Moon 10° minus Sun 350°: −340° becomes 20°, placing it in the second tithi.

Four worked examples: similar appearance, different tithi

The following positions are teaching inputs, not observations. Their absolute longitudes use one common reference. The phase labels describe the usual appearance around these elongations; they are not precise illumination-percentage calculations.

For the first row, 120° − 30° = 90°. Dividing 90 by 12 gives 7.5; taking the whole-number part and adding one gives 8. For the last row, 300° − 30° = 270° and the same operation gives 23. Subtracting 15 gives the eighth tithi of Krishna Paksha.

Thus both quarter phases correspond to Ashtami, but in different pakshas. The second and third examples both have a gibbous appearance, with more than half the disk lit. Their tithis are Shukla Ekadashi and Krishna Shashthi. We have not assigned an exact equal percentage to them: precise illumination uses the actual Sun–Earth–Moon geometry.

A full-Moon instant is not the entire Purnima tithi

The USNO phase definitions place the four primary phases at apparent geocentric longitude differences of 0°, 90°, 180° and 270°: new Moon, first quarter, full Moon and last quarter. These names can identify particular instants. Tithis occupy intervals between successive 12° boundaries.

Using an interval rule that includes the starting boundary and excludes the ending one, Purnima spans 168° up to 180°. The full-Moon instant at exactly 180° marks its ending boundary and the start of Krishna Pratipada. Similarly, the new-Moon instant at 0° after wrapping marks the transition from Amavasya to Shukla Pratipada.

This is a rule for assigning a mathematical boundary, not an instruction to rename a festival date. A calendar may prominently label the day containing the full-Moon event, while a panchang displays Purnima with its ending time. Read what each label measures before concluding that the two disagree.

Why place, clock display and moonrise still matter

A geocentric phase or tithi boundary is an instant that can be expressed in different time zones. A different clock label does not by itself imply a different astronomical event. The local date can change when that instant is converted to another zone.

Assigning an observance to a civil date also involves the relevant calendar rule and, often, local sunrise or another specified period. Our tithi and calendar-date guide and sunrise guide explain that next step. Do not select a vrat date from a rounded illumination percentage.

Moonrise answers a separate horizon question. A phase label does not guarantee that the Moon is above your horizon at the time you look. Check local moonrise and moonset, including the displayed date when an event is after midnight.

How to compare a Moon app with a panchang

1. Match the instant and time zone, rather than comparing two unspecified “today” readings.
2. Identify whether each field is an illumination percentage, a phase instant, a tithi interval or a day label.
3. Check waxing or waning before comparing a similar-looking disk.
4. Retain unrounded positions or times when checking a boundary.
5. Use the applicable observance rule for the final date decision.

The Amavasya and Purnima calendar brings the date question into view. The methodology page explains the calculation context. Keep the example angles above as learning aids rather than substituting them for a date-specific result.

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