Srimad-Bhagavatam: Canto 5 - Chapter 1 - Verse 6
Sanskrit:
अथ च यावतार्धेन नभोवीथ्यां प्रचरति तं कालमयनमाचक्षते ॥ ६ ॥
ITRANS:
atha ca yāvatārdhena nabho-vīthyāṁ pracarati taṁ kālam ayanam ācakṣate.
Translation:
Thus the time the sun takes to rotate through half of outer space is called an ayana, or its period of movement [in the north or in the south].