How Tithi is Calculated: The Solar-Lunar Geometry Behind Hindu Lunar Days
Discover why a Tithi varies from 19 to 26 hours and how a 12-degree lunar separation defines the Panchang.
By Vedic Astronomical Research Team · Updated August 2026
Unlike 24-hour civil solar days, a Tithi is determined by the exact angular longitudinal distance between the Sun and Moon. Learn the mathematics of Shukla and Krishna Paksha.
The Definition of a Tithi
In Hindu astronomy (Jyotisha), a Tithi is defined as the time duration in which the Moon increases its longitudinal distance from the Sun by exactly 12 degrees. Because a full lunar orbital circle is 360 degrees, a complete synodic lunar month (from New Moon to New Moon) contains exactly 30 Tithis (360° / 12° = 30).
- Shukla Paksha (Waxing Phase): Tithis 1 to 15, ending in Purnima (Full Moon, 180° separation).
- Krishna Paksha (Waning Phase): Tithis 16 to 30, ending in Amavasya (New Moon, 360° / 0° separation).
Why Tithi Duration Varies (19 to 26 Hours)
Because the orbits of both Earth around the Sun and the Moon around Earth are elliptical rather than perfectly circular, their relative orbital speeds change according to Kepler’s laws of planetary motion. When the Moon is near perigee (closest to Earth), it moves faster, covering 12 degrees in as little as 19 hours. When near apogee (farthest), it moves slower, extending the Tithi to nearly 26 hours.
Kshay Tithi and Vriddhi Tithi Explained
In daily Panchang civil usage, a Tithi is assigned to a date based on which Tithi is active at the exact moment of Sunrise (Surya Udaya). - Kshay Tithi (Skipped Tithi): A Tithi that begins after sunrise and ends before the next sunrise, so it never touches a sunrise. - Vriddhi Tithi (Extended Tithi): A Tithi that spans across two consecutive sunrises.
Frequently Asked Questions
Q: Why do festival dates sometimes span across two calendar days?
Because a Tithi can begin at any hour of the day or night (e.g. 3:15 PM), religious festivals depend on whether a specific ritual requires the Tithi at sunrise, midday (Madhyahna), or night (Nishita Kaal).