Chatushpada चतुष्पाद

Chatushpada is one of the four fixed karanas, each of which occurs exactly once per lunar month. It sits in the closing half-tithis of the month, just before the new moon. Being fixed, it cannot be moved around in muhurat selection — it either falls in your window or it does not.

A karana is six degrees of the Moon's elongation from the Sun — exactly half a tithi. Every tithi therefore contains two karanas, and every tithi boundary is also a karana boundary. Our engine computes the two independently and asserts they land on the same instant, which is how we know the boundary search is right.