Glossary · concept
Tithi
Also written lunar day
A tithi is a lunar day, and it is not the same length as a calendar day. It is defined by angle: one tithi is the time the moon takes to pull 12° ahead of the sun. Thirty of those make a full lunar month, from new moon back to new moon.
Because the moon’s speed varies, a tithi can run anywhere from about 19 to 26 hours — so it can begin and end at any hour, and the tithi running at sunrise is the one that names the day. Within each fortnight they are counted Pratipada (1st), Dvitiya (2nd), Tritiya (3rd) … up to the fifteenth: Purnima (full moon) or Amavasya (new moon).
Most festivals are fixed to a tithi, not a date — which is why Holi is always the Phalguna full moon even as its Western date moves.
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