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].