What is Tithi?

A tithi (Nepali: तिथि) is the unit of a lunar day in the Hindu lunisolar calendar, and it is defined not by the clock but by the relative motion of the Moon and the Sun. Specifically, one tithi is the time the Moon takes to gain 12 degrees of longitudinal elongation on the Sun. Because a full circle of 360 degrees divided into 12-degree steps yields 30 steps, every synodic (lunar) month contains exactly 30 tithis, completing one full cycle from one new moon to the next.

The 30 tithis are split into two fortnights, called paksha. The Shukla Paksha (bright fortnight) covers the waxing Moon, running from just after the new moon up to the full moon as the Moon's lit face grows. The Krishna Paksha (dark fortnight) covers the waning Moon, from just after the full moon down to the next new moon. Each fortnight holds 15 tithis. The fifteenth tithi of the bright fortnight is Purnima (full moon) and the fifteenth of the dark fortnight is Amavasya (new moon); the intervening days are numbered Pratipada (1st), Dwitiya (2nd), Tritiya (3rd), Chaturthi, Panchami, Shashthi, Saptami, Ashtami, Navami, Dashami, Ekadashi, Dwadashi, Trayodashi and Chaturdashi.

Unlike a solar day of fixed 24 hours, a tithi varies in length because the Moon moves at an uneven speed along its elliptical orbit. A tithi can run from roughly 19 hours 59 minutes to about 26 hours 47 minutes, averaging close to 23 hours 37 minutes. This variability is the source of much of the apparent complexity of Hindu calendar dates, and it is why a printed Nepali patro (almanac) lists the exact clock time at which each tithi begins and ends rather than treating a tithi as a tidy calendar box.

Because a tithi can start and end at any hour, calendars need a rule to decide which single tithi labels a given civil day. The convention is the udaya tithi rule: the tithi that is in force at the moment of local sunrise is treated as the tithi of the whole day, even though that tithi may end later that morning and a new one begin. This is why two people in different cities can, on rare occasions, record slightly different tithis for the same date, and why almanacs are computed for a specific location such as Kathmandu.

The sliding mismatch between the roughly 23.6-hour average tithi and the 24-hour civil day produces two special situations. A kshaya tithi (omitted or lost tithi) occurs when a tithi begins after one sunrise and ends before the next, so it never holds any sunrise and is skipped from the day-count. The opposite, a vriddhi or adhika tithi (repeated tithi), occurs when a single tithi spans two consecutive sunrises and therefore gives its name to two calendar days in a row. Both are normal features of the system, not errors.

A tithi is further halved into two karanas, the smaller reckoned unit; there are 11 karana types (4 fixed and 7 movable) that cycle through the month. Together with the tithi, the karana forms part of the panchanga, the five-limbed Hindu almanac whose other elements are the vara (weekday), nakshatra (lunar mansion) and yoga (a Sun-Moon angular combination). The tithi is the first of these five limbs and the one on which most religious dating depends.

Nepal's major festivals are dated by tithi, not by the Gregorian date, which is why they fall on a different English date each year while always landing on the same lunar day. Dashain, the country's biggest festival, runs across roughly a fortnight: it begins on Ghatasthapana at the Pratipada of the bright fortnight of Ashwin, its central Vijaya Dashami (tika) day falls on the tenth tithi (Dashami), and the celebrations close on Kojagrat Purnima, the full-moon tithi. The festival is therefore anchored end-to-end to specific tithis rather than to fixed dates.

Many other Nepali observances are pinned to a single named tithi. Full-moon (Purnima) tithis carry festivals such as Holi/Phagu Purnima, Buddha Jayanti (Baishakh Purnima) and Janai Purnima/Raksha Bandhan. Krishna Janmashtami falls on the Ashtami (eighth tithi) of the dark fortnight; Haritalika Teej is observed on the Tritiya of the bright fortnight of Bhadra; Maha Shivaratri falls on the Chaturdashi of the dark fortnight of Falgun; and the Ekadashi (eleventh) tithis are widely kept as fasting days. New-moon (Amavasya) tithis likewise mark observances such as the Laxmi Puja night of Tihar.

To find a festival's English date in any given year, an almanac computes when the relevant tithi is current at sunrise in Nepal and assigns the festival to that civil day. This is also why a festival can occasionally appear to shift by a day between calendars or be observed on two days where a tithi is repeated or omitted — the underlying lunar event is fixed, but its projection onto the 24-hour civil day is not.

The tithi belongs to the Hindu lunisolar calendar, the framework underlying Nepal's official Bikram Sambat (Vikram Samvat) system, which combines lunar months with a solar sidereal year. The astronomical rules for computing tithis — based on the mean and true motions of the Sun and Moon — are set out in classical Indian astronomical treatises such as the Surya Siddhanta, and modern almanacs refine these using high-precision ephemerides, which can differ from the traditional mean-motion figures by several minutes.

A lunar (synodic) month averages about 29.5 days, so twelve lunar months total roughly 354 days, about eleven days short of the roughly 365-day solar year. To keep lunar months aligned with the seasons, the calendar inserts an intercalary month, the adhik maas (extra month), roughly every 32–33 months. Two reckoning traditions also coexist: the amanta system ends each lunar month at the new moon, while the purnimanta system ends it at the full moon, so the same Krishna-Paksha tithi can carry different month names in different regions — a detail that matters when dating festivals and historical records.

Because the traditional ritual calendar names the days of each fortnight by tithi (Pratipada, Dwitiya, and so on up to Purnima or Amavasya) rather than by a running 1-to-30 count, the tithi is not merely an astrological refinement but the structural backbone of how religious time is kept in Nepal. Almanacs, temple schedules, fasting days and auspicious-time (sait/muhurta) selections are all expressed through it.

The 15 tithis of a paksha are traditionally grouped into five recurring classes, each appearing three times per fortnight, and each carrying a customary character used when choosing auspicious times. The classes are Nanda (joy: tithis 1, 6, 11), Bhadra (good/fortunate: 2, 7, 12), Jaya (victory: 3, 8, 13), Rikta (empty: 4, 9, 14) and Purna (full/complete: 5, 10, 15). In popular practice the Nanda tithis are favoured for celebrations, Bhadra for travel and study, Jaya for competitive undertakings and Purna for completing things, while the Rikta tithis are usually avoided for starting important work.

Individual tithis also have presiding deities and ritual associations in tradition — for example Chaturthi is linked with Ganesh, Ekadashi with fasting and Vishnu, and Purnima and Amavasya with major pujas. These associations are devotional conventions of the Hindu astrological tradition rather than astronomical facts, and they vary by region, sect and almanac.

It is important to distinguish the tithi (a lunar day, fixed by the 12-degree Moon-Sun elongation) from related calendar terms: the vara is the ordinary seven-day weekday; the nakshatra is the lunar mansion (the constellation the Moon occupies); the paksha is the fortnight in which a tithi sits; and the karana is the half-tithi. A common point of confusion is treating a tithi as a fixed 24-hour day — it is not, and that single difference explains kshaya (skipped) and vriddhi (repeated) tithis and the year-to-year movement of festivals.