Tithi
A tithi is a lunar day, the time the Moon takes to move 12° ahead of the Sun. Thirty tithis make one lunar month, and the tithi at sunrise names the Hindu day.
What a tithi is
A tithi is one of the thirty divisions of the lunar month. It is defined by the angle between the Moon and the Sun as seen from Earth: each time that angle grows by another 12 degrees, a new tithi begins. Because the Moon’s speed varies, a tithi can last anywhere from about 19 to 26 hours, so it does not line up with the civil day.
The fifteen tithis of the waxing half (Shukla Paksha) run from Pratipada to Purnima, the full moon. The fifteen of the waning half (Krishna Paksha) run from Pratipada again to Amavasya, the new moon.
Which tithi is “today”
In the panchang the day takes the tithi that is running at local sunrise. That is why the same date can show different tithis in different cities, and why a tithi that begins after sunrise and ends before the next sunrise (a kshaya tithi) is skipped in the day count, while one that spans two sunrises (a vriddhi tithi) is counted twice.
Festivals and vratas use more precise rules: Ekadashi is kept on the day the tithi prevails at sunrise, Janmashtami on the day Ashtami prevails at midnight, Karva Chauth on the day Chaturthi prevails at moonrise.
The thirty tithis
Pratipada, Dwitiya, Tritiya, Chaturthi, Panchami, Shashthi, Saptami, Ashtami, Navami, Dashami, Ekadashi, Dwadashi, Trayodashi, Chaturdashi, and then Purnima or Amavasya. Chaturthi, Navami and Chaturdashi (the Rikta tithis) are avoided for new beginnings; Dwitiya, Tritiya, Panchami, Saptami, Dashami, Ekadashi and Trayodashi are generally favourable.
FAQ
Why does the tithi change at an odd time like 07:07?
Because the tithi is astronomical: it ends the instant the Moon–Sun angle crosses the next 12° mark, wherever the Moon happens to be in the sky. Hastro computes that instant to the second for your location’s time zone.
How long is a tithi?
On average 23 hours 37 minutes, but it ranges from roughly 19 to 26 hours depending on how fast the Moon is moving in its orbit.
Can two tithis fall on one day?
Yes. When a tithi begins after one sunrise and ends before the next, it never “owns” a sunrise and is called a kshaya (lost) tithi. Fasts tied to it are observed on the day that contains it.