Surya Siddhanta
अतो दिनक्षपे तेषाम् अन्योन्यम् हि विपर्ययात् । अहोरात्रप्रमाणम् च भानोर् भगणपूरणात् ॥
Hence are their night and day mutually opposed to one another ; and the measure of the day and night is by the completion of the sun's revolution.
english translation