Manava Dharmashastra 1.67

1.67

varṣaṃ manuṣyāṇāṃ dvādaśamāsāstadekamahorātraṃ devānāmḥ tayośca vibhāga-udagayanadakṣiṇāyanābhyām | tatrodīcīṃ diśamabhipratiṣṭhamānasyādityasya ṣaṇmāsā udagayanaṃ bhavati | 'ayanaṃ' gamanamadhiṣṭhānaṃ vā | tasyāṃ diśi ṣaṇmāsānāditya udeti | tataḥ parāvṛttasya dakṣiṇāyanam | tathā hi dakṣiṇāṃ diśamākramyodīcīṃ hitvā hyudayaṃ karoti |

One ‘year’ (of men) forms the ‘Day and Night’ of the Gods; and the division of these is that the ‘Northern course’ is the ‘Day,’ and the ‘Southern Course’ the ‘Night.’—(67)

Medhātithi’s commentary (manubhāṣya):
‘One year,’ of men,—i.e., twelve months,—‘forms the Day and Night of the Gode.’—The division of these is by means of the ‘Northern Course’ and the ‘Southern Course’; the six months during which the Sun moves towards the North is the ‘Northern Course,’ ‘Udagayanam’; the term ‘ayana’ standing for moving or occupying; the meaning being that during the six months the Sun rises towards that particular quarter. Turning hack from that, there comes the ‘Southern course’; during these six months the Sun abandons the Northern quarters and rises towards the South.—(67)