Surya Siddhanta
ग्रीष्मे तीव्रकरो भानुर् न हेमन्ते तथाविधः। कियती तत्करप्राप्तिर् मानानि कति किम् च तैः॥
Why are the rays of the sun hot in the summer, and not so in the winter? how far do his rays penetrate? How many modes of measuring time (mana) are there? and how are they employed?
english translation