Matsya Purana — Solar–Lunar Motions
शरद्वसन्तयोर्मध्यं विषुवं तु विधीयते आलोकान्तः स्मृतो लोको लोकाच्चालोक उच्यते //
śaradvasantayormadhyaṃ viṣuvaṃ tu vidhīyate ālokāntaḥ smṛto loko lokāccāloka ucyate //
Viṣuva (the equinox) is fixed as the midpoint between autumn (śarad) and spring (vasanta). The realm whose boundary is light is remembered as Loka; and beyond Loka is called Aloka, the region without light.
It does not describe pralaya directly; it defines cosmic geography in terms of light—‘Loka’ as the illuminated domain and ‘Aloka’ beyond it—ideas often used in Purāṇic cosmology that frames cycles of manifestation and obscuration.
By marking viṣuva (equinox) as a calendrical midpoint, it supports dharmic timekeeping—useful for a king’s public ritual scheduling and a householder’s observances (vratas, śrāddha timings, seasonal rites) aligned with sanctioned seasonal transitions.
The verse provides a timing principle: equinox as a key seasonal junction. In Matsya Purana–style ritual planning (and often Vastu-related muhurta selection), such junctions guide auspicious scheduling for consecrations, renewals, and seasonally-governed rites.