Surya Siddhanta
तासाम् उपरिगो याति विषुवस्थो दिवाकरः। न तासु विषुवच्छाया नाक्षस्योन्नतिर् इष्यते॥
The sun travels on a tract above them during the equinoxes, hence they don’t have equinoctial shadow (vishuvacchäyä) nor is there any no elevation of the axis (aksonnati).
english translation