Surya Siddhanta
तुलादौ द्युनिशोर् वामम् क्षयवृद्धी तयोर् उभे । देशक्रान्तिवशान् नित्यम् तद्विज्ञानम् पुरोदितम्॥
In the half-revolution beginning with Libra, both the deficiency and excess of day and night in the two hemispheres are the opposite of this : the method of determining them; which is always dependent upon situation (deqa) and declination, has been before explained.
english translation