Matsya Purana — Narasimha’s Victory over Hiraṇyakaśipu and the Catalogue of Apocalyptic Omens
यदा च सर्वभूतानां छाया न परिवर्तते अपराह्णगते सूर्ये लोकानां युगसंक्षये //
yadā ca sarvabhūtānāṃ chāyā na parivartate aparāhṇagate sūrye lokānāṃ yugasaṃkṣaye //
And when, for all beings, the shadow no longer shifts—though the sun has moved into the afternoon—then, at the end of the age, the worlds approach dissolution.
It gives an omen of yuga-ending dissolution: a breakdown of ordinary cosmic order, shown by shadows ceasing to move even as the sun progresses—signaling the worlds’ approach to pralaya.
Indirectly, it frames dharma within impermanence: rulers and householders should govern, give, and perform rites with awareness that worldly stability is time-bound and can collapse at yuga’s end.
No direct Vāstu rule is stated; the verse functions as a calendrical-cosmic warning sign, relevant to ritual timing only insofar as it marks abnormality in the sun’s course and worldly order.