SB 5.22.6

atha ca yāvatārdhena nabho-vīthyāṁ pracarati taṁ kālam ayanam ācakṣate.
atha — now; ca — also; yāvatā — by as long as; ardhena — half; nabhaḥ-vīthyām — in outer space; pracarati — the sun moves; tam — that; kālam — time; ayanam — ayana; ācakṣate — is said.
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].