Matsya Purana — Solar–Lunar Motions
वर्धत्यतो ह्रसत्येव अयने दक्षिणोत्तरे अहस्तु ग्रसते रात्रिं रात्रिस्तु ग्रसते अहः //
vardhatyato hrasatyeva ayane dakṣiṇottare ahastu grasate rātriṃ rātristu grasate ahaḥ //
Thus, in the two solstitial courses—southern and northern—the lengths of day and night alternately increase and decrease: the day indeed “swallows” the night, and the night in turn “swallows” the day.
It does not describe Pralaya directly; it explains the regular cosmic order of time—how day and night alternately expand and contract across the sun’s northern and southern courses.
By defining the rhythm of day and night across Uttarayana and Dakshinayana, it supports correct timing for governance, agriculture, fasting, and domestic rites—since many duties and vows are scheduled according to seasonal daylight and the sun’s course.
Indirectly, it underpins ritual calendrics (muhūrta and seasonal observances): knowing the ayanas helps choose auspicious periods for yajña, vrata, and temple consecration schedules, even though no specific Vastu rule is stated in this verse.