Matsya Purana — Solar–Lunar Motions
तस्मात्संध्येति तामाहुर् उषाव्युष्टैर्यथान्तरम् उषा रात्रिः स्मृता विप्रैर् व्युष्टिश्चापि अहः स्मृतम् //
tasmātsaṃdhyeti tāmāhur uṣāvyuṣṭairyathāntaram uṣā rātriḥ smṛtā viprair vyuṣṭiścāpi ahaḥ smṛtam //
Therefore, the learned call the interval between uṣā (dawn) and vyuṣṭi (full daybreak) “sandhyā” (the sacred twilight junction). The Brahmins remember uṣā as belonging to the night, while vyuṣṭi is held to belong to the day.
This verse does not discuss pralaya; it defines sandhyā as a measurable transition between night and day, emphasizing correct ritual timekeeping rather than cosmological dissolution.
By defining the precise boundary of sandhyā, it supports correct scheduling of daily duties such as sandhyā-vandana, japa, and other nitya-karmas—disciplines expected of householders and upheld by righteous rulers as part of public dharma.
The significance is ritual: it establishes when sandhyā rites should be performed by identifying uṣā as night and vyuṣṭi as day, making the in-between interval the proper sandhyā period.