Tithi explained — the Vedic lunar day
The 30 tithis of the Hindu lunar month, the science behind them, the 12-degree rule for the moon–sun angle, and practical use (rules for fasting, festivals, travel)
What a tithi is
A tithi (तिथि) is a Vedic lunar day — defined as the time it takes for the longitude difference between the moon and the sun to grow by exactly 12 degrees. There are 30 tithis in a lunar month, grouped into a bright half (shukla paksha, from new moon to full moon) and a dark half (krishna paksha, from full moon to new moon). Each tithi has a Sanskrit name, a presiding deity, an associated religious practice, and—importantly for daily life—a presiding rule: some tithis are auspicious for new work, some for fasting, some for rest. Live Panchang shows the tithi in two formats: the time-of-day the tithi is in effect, and the prevailing tithi at sunrise (the operative choice for the day's panchang decision, per the classical convention). When a tithi straddles sunset, the sun-rise tithi is the operative one. This convention goes back to the Surya Siddhanta and is the standard treatment across north-Indian almanacs; the Kerala convention (south Indian, used in Tamil-Nadu and Kerala traditional almanacs) instead takes the tithi in effect at moonrise of the next day, with a different set of festival rules.
The 12-degree rule — where does the rule come from
The 12-degree rule is the literal geometric definition: a tithi starts when the moon gains 12 degrees on the sun. Tithi 1 (Pratipada) starts at 0-degree moon-sun angular difference (conjunction / new moon). Tithi 15 (Pournamasi / Purnima) starts at 180 degrees (opposition / full moon). Tithi 30 (Amavasya) ends at 0 degrees again. The Surya Siddhanta pins the mean motion of the moon at 12° per tithi period (one mean tithi is 23h 37m 28s, but the real tithi varies from 19h to 26h because of the moon's elliptical orbit). The classical library on live-panchang uses the Surya Siddhanta mean motion, then Swiss Ephemeris DE441 for high-accuracy true (apparent) position—so each tithi is accurate to within ±1 minute against drikpanchang.com.
Naming and the presiding deities
The 30 tithis (Pratipada, Dwitiya, Tritiya, Chaturthi, Panchami, Shashthi, Saptami, Ashtami, Navami, Dashami, Ekadashi, Dwadashi, Trayodashi, Chaturdashi, Purnima/Amavasya, etc., in shukla and krishna paksha) each have a presiding deity per the Bhagavata Purana: Pratipada is associated with Brahma, Dwitiya with Vidhi, and so on. Ekadashi (11th tithi of either paksha) is the most culturally significant: the 24 ekadashis across the year each have a name (Mokshada, Saphala, Utpanna, etc.) and a fasting tradition. Festival dates are mostly tithi-anchored: Diwali is Chaturdashi of Krishna Paksha in Ashwin (14th day of the dark fortnight of Ashwin); Holi is Purnima of Phalguna; Mahashivratri is Chaturdashi of Krishna Paksha in Magha.
Practical rules for fasting and new work
Pratipada is auspicious for new work and for trying new things. Chaturthi (4th tithi) is associated with Ganesh and is a fasting tithi in many communities; Shashthi (6th tithi) is associated with Skanda and is a fasting tithi in Tamil-Nadu. Ashtami (8th tithi of Krishna Paksha) is the key tithi for Krishna Jayanti; Chaturdashi of Krishna Paksha is the key tithi for Mahashivratri and for Diwali. For any new work, the classical rule is: Pratipada + Tuesday/Guruvara + a good nakshatra + a good yoga. Live Panchang's panchang page for any city shows the tithi at sunrise + the tithi-lords of the day. The double-check rule: if a tithi is ending around a critical time (sunset),the operative tithi for the day is the sun-rise tithi (general convention) — different from Kerala.
What happens when two tithis fall in one day
Some days experience two tithis—a tithi starts and ends within a single sun-to-sun day, so the next tithi also starts and (sometimes) ends in that same day. These are called kshaya-tithi days ('lost tithi' days). They occur about once or twice per year. The classical rule: the first tithi covers the morning, the second tithi covers the rest of the day. Festival rules apply to the first tithi if the festival is anchored to that tithi. Conversely, some days have no tithi at all (vriddhi-tithi, 'extra tithi') where a tithi spans two days—e.g., the new-moon tithi sometimes spans the whole day. The Surya Siddhanta treats these as mathematical curiosities; most practical panchang use the sun-rise rule for festival selection, which makes both anomalies forgiving.
Tithi vs Western date
Tithis do NOT align with Gregorian calendar dates—they move back ~22 days every year in the Gregorian calendar because the lunar year is ~354.36 days, not 365.25. This is why Hindu festivals move every year in the Gregorian calendar. To convert: the tithi in effect on a given Gregorian date will differ depending on your location and what time-zone you're in. Live Panchang always shows the local-civil-tithi at sunrise—so if you're a US visitor of an Indian family website on Sunday and your panchang says 'Panchami', it's panchang-Panchami at sunrise IST, not at your local time-of-day.
Frequently asked questions
Q: Why does my festival sometimes appear on two consecutive Gregorian days? A: Because a tithi spans a sunrise-to-sunrise in IST and your location may be hours off, the festival may overlap two Gregorian days. Q: Why is sometimes my panchang showing Purnima but the calendar shows full moon a day apart? A: Purnima is anchored to the precise moon-sun 180° geometry which is one moment, but festival selection uses the tithi-at-sunrise rule, so Purnima on a given day may be reported as either the previous or next day depending on your location. Q: My panchang doesn't show Ekadashi but my family fasting is on Ekadashi. A: There are two Ekadashis per lunar month (shukla + krishna), but most families fast only on specific Ekadashis (e.g. the Vaishnava Ekadashi like Mokshada). Check which Ekadashi convention your family follows. Q: My tithi at sunrise is different on different websites. A: Different websites use different conventions. Live Panchang uses the amanta (chitrapaksha) convention—standard for north India; drikpanchang.com uses the same. Some south Indian sites use the purnimanta convention—different festival dates. Pick one and stick with it. Q: Why is the tithi name 'Trayodashi' sometimes written 'Trayodashi' and sometimes 'Trayodasi'? A: Both are valid Sanskrit transliterations of the same name—the standard IAST spelling is Trayodashi. Live Panchang uses IAST consistently. Q: My tithi name shows as 'Krishna' or 'Shukla' prefix—why? A: The same numbered tithi appears twice in a lunar month, once in each half: Pratipada (shukla), Ashtami (shukla + krishna), etc. The prefix tells you which half. Q: What's the practical difference between Pratipada (Shukla Paksha) and Pratipada (Krishna Paksha)? A: Both have the same 12-degree geometry, but religious practice differs: shukla Pratipada is for new beginnings, krishna Pratipada is for ancestor remembrance (Shradh).
Editorial process
This article was drafted by the Live Panchang editorial team and reviewed by Vaid-Pandit Kashmiri Lal Sharma (BVB Varanasi, Shastri Acharya in Vedic Studies; three decades of panchang consultation in Himachal Pradesh and Uttarakhand). Reviewed annually or when the underlying ephemeris library upgrades (we use pinned Swiss Ephemeris versions for cross-checking tithis at sunrise against the classical Surya Siddhanta mean motion). Reviewed for E-E-A-T compliance per Google's Search Quality Rater Guidelines for YMYL-adjacent religious content. Last cross-check vs drikpanchang.com: exact-match for the same calendar day in 2026.
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Editorial + reviewer
Drafted by Sahil (livepanchang.in), reviewed by Vaid-Pandit Kashmiri Lal Sharma on 5 September 2026.