Tithi

What is a tithi?

A tithi is a lunar day — the fundamental unit of the Hindu lunar calendar. It is defined purely by the angle between the Moon and the Sun: one tithi is the time the Moon takes to move 12° ahead of the Sun. Since a full circle is 360°, thirty of these 12° steps complete one lunar month.

The key idea, and the source of nearly every practical complication, is that a tithi measures angle, not time. The clock is only along for the ride. When the Moon is moving quickly relative to the Sun, it covers those 12° in as little as about 20 hours; when it is slow, a single tithi can stretch past 26 hours. So a tithi is almost never a neat 24-hour day, and it rarely begins at sunrise or ends at midnight.

Paksha: the two fortnights

A lunar month splits into two pakshas of fifteen tithis each:

• Shukla Paksha — the bright, waxing half, from the new moon to the full moon. The Moon grows fuller each night.
• Krishna Paksha — the dark, waning half, from the full moon back to the new moon. The Moon thins each night.

The two fortnights are bookended by the two syzygies: Purnima (full moon) closes Shukla Paksha, and Amavasya (new moon) closes Krishna Paksha. Every other tithi name — Pratipada, Dwitiya, and so on — appears once in each paksha, so you must always say which: Krishna Ashtami and Shukla Ashtami are two different days, often two very different observances.

The thirty tithis

The fifteen names repeat across both pakshas. What gives each a character is its nature, which follows a fixed five-fold cycle — Nanda, Bhadra, Jaya, Rikta, Purna — that repeats three times within every fortnight.

The fifteenth tithi is the one that differs by paksha: it is Purnima at the end of the waxing half and Amavasya at the end of the waning half.

The five natures, and why Rikta matters

The Nanda–Purna cycle is not decorative; it is one of the first filters a muhurat selection applies.

• Nanda (1, 6, 11) — “delight.” Good for celebration, festivity, new ventures.
• Bhadra (2, 7, 12) — “auspicious.” Good for health, routine, steady work.
• Jaya (3, 8, 13) — “victory.” Good for competition, confrontation, effort.
• Rikta (4, 9, 14) — “empty.” Traditionally avoided for weddings, house-warming, travel, and other auspicious beginnings; often reserved instead for demolition, settling disputes, or actions where “emptying” is the point.
• Purna (5, 10, 15) — “full.” Good for completion, journeys, and anything you want to see through to fullness.

Treat these as an interpretive convention, not an iron law — the prevailing nakshatra, yoga, weekday and the specific work can all override a tithi’s default character. They are a starting filter, not a verdict.

Why a tithi isn’t 24 hours

Because a tithi is 12° of elongation (the Moon’s lead over the Sun), its duration depends on how fast that lead is opening. Two things vary it:

1. The Moon’s speed swings between roughly 12° and 15° per day as it moves between apogee (farthest, slowest) and perigee (nearest, fastest).
2. The Sun’s apparent speed varies slightly too, as the Earth moves between aphelion and perihelion.

Combine them and the Moon’s relative gain on the Sun ranges from about 10.8° to 14.4° per day. Dividing the fixed 12° step by that changing rate gives a tithi length of roughly 20 to 27 hours, with a mean of about 23 hours 37 minutes — just under a civil day.

That “just under” is what produces two special cases:

• Kshaya tithi (decayed): a short tithi that begins after one sunrise and ends before the next, so no sunrise falls inside it. It effectively “skips” a day on the calendar.
• Vriddhi tithi (extended): a long tithi still current at two consecutive sunrises, so it “repeats.” Some almanacs mark the second as adhika.

Neither is an error — both are the direct, expected consequence of measuring days by angle rather than by clock.

Reading a tithi in AstroShruti

Open the Panchang for any date and city and the tithi is the first limb shown, written as paksha + tithi name + end time. An illustrative reading looks like this:

Krishna Paksha · Chaturthi — until 14:32, then Panchami

Read left to right, that tells you: the Moon is in the waning fortnight; the current lunar day is the fourth (a Rikta tithi — note that before fixing an auspicious start); and the astronomical tithi ends at 2:32 pm local time, after which Panchami begins. Because the boundary is astronomical, that end time is specific to the date and location you chose — a city further east will see the same tithi end at a different clock time.

Open the Panchang for any date and location to see the exact tithi, its end moment, and the rest of the day’s limbs computed for your location.

Which tithi “counts” for a vrat or festival

This is where most confusion starts. A festival is assigned to a civil day, but a tithi is a floating astronomical window that seldom fills one neatly. The tradition resolves this with a vyapini (“prevailing”) rule: the observance falls on the day when its tithi is present at a defined moment.

• Many vrats use the tithi prevailing at sunrise.
• Some use midday (madhyahna), sunset (pradosh, as with Trayodashi vrats), or midnight (nishita, as with Krishna Janmashtami).
• Ekadashi, Chaturthi (Ganesha), Purnima and Amavasya each carry their own well-known conventions.

AstroShruti computes the astronomical tithi and its exact boundaries; the ritual choice of which moment governs a specific vrat belongs to tradition, and can legitimately differ between regional almanacs and sampradayas.

Common mistakes

• Treating the tithi as the date. “Today is Ekadashi” usually means Ekadashi is current at sunrise — it may well have ended by lunchtime.
• Forgetting the paksha. Shukla Ashtami and Krishna Ashtami share a name and little else.
• Expecting 30 tithis to fill 30 days. A lunar month is about 29.5 civil days, so tithis and dates drift apart, producing the kshaya and vriddhi cases.
• Assuming one universal rule for festivals. The sunrise rule is common but not exclusive; the correct moment depends on the observance.

What AstroShruti computes — and what it doesn’t

AstroShruti calculates the astronomical tithi: the Moon–Sun elongation for any date, time and location, the tithi’s name and paksha, and its exact end moment, using the same engine that drives the charts. It also flags kshaya and vriddhi tithis where they occur.

What it does not do is legislate ritual. Which civil day a particular vrat falls on — the vyapini moment, the handling of a tithi that straddles two days, and the regional variations that follow — is a tradition-level judgment. We give you the exact astronomy so that judgment rests on solid ground, but the observance rule remains yours (or your family priest’s) to apply.

Related concepts

• Nakshatra — the Moon’s lunar mansion, the next limb of the panchang.
• Karana — the half-tithi, of which there are two to each tithi.
• Masa — the lunar month the tithis build up into.
• Vara — the weekday, which runs from sunrise to sunrise like the panchang itself.