Bhagavad Gita
8.25

धूमो रात्रिस्तथा कृष्णः षण्मासा दक्षिणायनम् । तत्र चान्द्रमसं ज्योतिर्योगी प्राप्य निवर्तते ॥

dhūmo rātristathā kṛṣṇaḥ ṣaṇmāsā dakṣiṇāyanam | tatra cāndramasaṃ jyotiryogī prāpya nivartate ||

Translation

Smoke, night, the dark fortnight of the moon, the six months of the southern course of the sun — departing in these, the yogi attains the lunar light and returns.

Interpretation

The path of darkness (or the path of smoke): associated with smoke, night, the dark lunar fortnight (krishna paksha), and the southern course of the sun (dakshinayana). The yogi who departs in these conditions attains the lunar realm — the realm of ancestors and heavenly enjoyment — but eventually returns to rebirth. The lunar light is beautiful but reflected, temporary; the solar path leads to the direct light of Brahman. In the deeper reading: the yogi who dies in a state of partial realization, still holding some residue of desire, follows this path.