Surya Siddhanta
ज्ञेया चरदलज्या च विषुवत् क्षितिजान्तरम् । कृत्वोपरि स्वकम् स्थानम् मध्ये क्षितिजमण्डलम्॥
And the sine of the sun's ascensional difference (caradala) is to be recognized as the interval between the equator (vishuvat) and the horizon. Having turned upward one's own place, the circle of the horizon is midway of the sphere.
english translation