A Tithi is a lunar day in the Hindu calendar. Unlike a solar (civil) day, it isn't a fixed 24-hour block — it's defined by the angular distance (elongation) between the Moon and the Sun as seen from Earth. One tithi spans exactly 12° of relative motion, so a full lunar month of 360° contains 30 tithis.

Because the Moon's orbital speed varies along its elliptical path, tithis don't have a fixed duration — they typically range from about 19 to 26 hours. This is why a tithi can start and end at different clock times each month, and occasionally spans zero or two sunrises.

Amavasya corresponds to the 30th tithi. This calculator computes the precise moment the Moon-Sun elongation crosses 348° (the start of the Amavasya tithi) using an astronomical bisection search against NASA JPL ephemeris data, then finds the matching new moon at 0°/360° elongation to mark the tithi's end.