Surya Siddhanta
विपरीतायनगतौ चन्द्रार्कौ क्रान्तिलिप्तिका । समास् तदा व्यतीपातो भगणार्धे तयोर् युतौ॥
When the moon and sun are upon opposite sides of either solstice, and their minutes of declination are the same, it is vyatipdta, the sum of their longitudes being a half-circle.
english translation