A Tithi is a lunar day defined as the period during which the angular distance between the Moon and the Sun increases by exactly 12 degrees. A full lunar month spans 360 degrees divided into 30 Tithis: 15 in the waxing fortnight (Shukla Paksha) ending in Pournami, and 15 in the waning fortnight (Krishna Paksha) ending in Amavasya.
☕ Think of it like a celestial battery indicator
Imagine the Moon as a giant cosmic battery in the night sky. On New Moon (Amavasya), the Moon and Sun sit at the same celestial longitude (0° separation). The Moon is completely dark, like a battery at 0%.
As the Moon orbits Earth eastward, it pulls away from the Sun by roughly 12° each day. With every 12° step (Prathama, Dvitiya, Tritiya, Chaturthi...), the visible illuminated crescent widens, reaching 100% full capacity at 180° directly opposite the Sun on Pournami (Full Moon).
During the second half of the month, the angle widens further from 180° to 360°, gradually depleting the visible light through 15 Krishna Paksha tithis until the cycle resets at Amavasya. This is why a Tithi is not a clock-bound human invention — it is the observable geometry of the heavens.
Tithi Facts vs. Common Misconceptions
- Exact geometric angle: 12° separation between lunar and solar geocentric longitudes
- Variable duration: lasts between 19 and 26 hours due to the Moon's elliptical orbit (Kepler's laws)
- Two distinct fortnights: Shukla Paksha (waxing crescent) and Krishna Paksha (waning crescent)
- Udaya Tithi rule: the Tithi active at local sunrise governs civil and festival observance for that day
- Tidal link: spring tides come within a day or two of Amavasya and Pournami, and neap tides near the quarter moons
- Not a 24-hour clock day: a Tithi can begin at any hour of the day or night and end at another
- Amavasya is not evil or cursed: traditionally a sacred threshold for ancestral memory, detox, and quiet rest
- Kshaya Tithi is not lost time: when a short tithi begins after sunrise and ends before the next, it is pure geometry
- Vriddhi Tithi is not double time: occurs when an unusually long tithi touches two consecutive sunrises
- Not rigid across cities: because sunrise occurs at different times in different longitudes, the Udaya Tithi can differ between cities
Frequently Asked Questions About Tithi
How is a Tithi calculated mathematically?
A Tithi is calculated by taking the Moon's celestial longitude, subtracting the Sun's celestial longitude, and dividing by 12 degrees: Tithi Number = floor((Moon_Longitude - Sun_Longitude) / 12) + 1. If the result is 1 to 15, it is Shukla Paksha; from 16 to 30, it is Krishna Paksha.
Why does a Tithi last between 19 and 26 hours?
The Moon orbits Earth in an ellipse, not a circle. When the Moon is closest to Earth (perigee), it moves faster across the sky (~15° per day), completing 12° of separation in as little as 19 hours. At apogee (farthest point), it slows down (~11.8° per day), stretching the Tithi up to 26 hours.
What is an Udaya Tithi?
In classical tradition, whatever Tithi is actively running at the exact moment of local sunrise (Surya Udayam) is designated as the Udaya Tithi. That Tithi name is associated with the entire civil day for festivals, fasting, and religious observances.
What are Shukla Paksha and Krishna Paksha?
Shukla Paksha is the bright, waxing fortnight starting the day after Amavasya and culminating in Pournami (Full Moon). Krishna Paksha is the dark, waning fortnight starting after Pournami and ending in Amavasya (New Moon).
Why do two calendars list different Tithis for the same festival?
If a Tithi transitions within a few minutes of sunrise, a city further east might see the transition occur after sunrise, while a western city sees it occur before sunrise. This geographic difference changes which Tithi prevails at sunrise. Calendars computed by different methods (Vakya or Drik) can also place the ending instant on either side of sunrise.
Deep Dives: The Mathematics of Lunar Phases
Explore the celestial mechanics of lunar velocity, synodic months, and phase divisions.
📐 The 12-Degree Formula and Synodic Cycle
A complete lunar phase cycle (from New Moon to New Moon) is called a synodic month, averaging 29.530588 days (29 days, 12 hours, 44 minutes). Dividing this 360° journey by 30 gives an average Tithi length of approximately 23 hours, 37 minutes, and 28 seconds.
Because the Sun also moves forward along the ecliptic by ~1° per day, the Moon must actually travel ~13.2° in space to gain 12° on the moving Sun.
🌗 Names of the 30 Tithis Across Both Fortnights
The 15 Tithis share identical numerical names in both fortnights, with only the final 15th phase changing:
- 1 to 5: Prathama, Dvitiya, Tritiya, Chaturthi, Panchami
- 6 to 10: Shashthi, Saptami, Ashtami, Navami, Dashami
- 11 to 14: Ekadashi, Dvadashi, Trayodashi, Chaturdashi
- 15 (Shukla): Pournami (Full Moon / 180° separation)
- 30 (Krishna): Amavasya (New Moon / 360° or 0° conjunction)
⚡ Kshaya and Vriddhi: Skipped and Double Tithis
Because civil days are measured from sunrise to sunrise (averaging 24 hours), while Tithis vary between 19 and 26 hours, the two cycles naturally slip against one another:
- Kshaya Tithi (Omitted): When a fast 19-hour Tithi starts 10 minutes after sunrise on Monday and concludes 10 minutes before sunrise on Tuesday, it never touches a sunrise. It is omitted from civil day names.
- Vriddhi Tithi (Extended): When a slow 26-hour Tithi starts right before sunrise on Wednesday and extends through Thursday's sunrise, two consecutive days take the same Udaya Tithi name.