8.17 सहस्रयुगपर्यन्तम् ending in a thousand Yugas (ages), अहः day, यत् which, ब्रह्मणः of Brahma, विदुः know, रात्रिम् the night, युगसहस्रान्ताम् ending in a thousand Yugas (ages), ते they, अहोरात्रविदः knowers of day and night, जनाः people.

Commentary:
Day means evolution or projection or manifestation of the universe. Night means involution of the universe or Pralaya. The worlds are limited or conditioned in time. Therefore they return again. The world of Brahma (Brahmaloka or Sattyaloka) is also transient, although it lasts for a thousand ages. When the four great Yugas have gone round a thousan times, it make a daytime of Brahma and when an eal number of Yugas pass again it makes a night. Those who can see and live through the day and night of Brahma can really know what is a day and what is a night.The Suryasiddhanta speaks of the same division of time.According to it Years

Kaliyuga (with its Sandhya and

Sandhyamsa) consists of 432,000

Dvapara Yuga (do) 864,000

Tretayuga (do) 1,296,000

Kritayuga (do) 1,728,000

Thus a Mahayuga consisting of

these four Yugas comprises 4,320,00071 such Mahayugas with an additional

Sandhya, at the close of 1,728,000

years make one Manvantara of 308,448,00014 such Manvantaras with another

Sandhya, at the close of 1,728,000

years constitute one Kalpa of 4,320,000,000

Two Kalpas make a day and night

of Brahma of 8,640,000,000360 such days and nights make one

year of Brahma consisting of 3,110,400,000,000100 such years constitute

His lifetime of 311,040,000,000,000The world is absorbed in the Avyakta or the Unmanifested or Mulaprakriti during the cosmic Pralaya (involution of the world). Just as the tree remains in a latent state in the seed, so also this whole universe remains in a latent state in a seedform in the Mulaprakriti during Pralaya. This is the night of Brahma. This is the cosmic night. Again the world is projected at the beginning of the Mahakalpa (evolution). There comes the cosmic dawn or cosmic day. This eternal rhythm of cosmic day and night (evolution and involution) is kept up in the macrocosm.Nothing that comes under this everrevolving wheel of cosmic day and night lasts for ever. That is the reason why the seers of the Upanishads, the sages of yore, lived in the transcendental Supreme being, the imperishable Self, the indestructible Purusha, the supreme goal of life, the highest end of man, which is beyond the cosmic day and night. Just as the seeds that are fried can hardly germinate, so also those who have attained to the imperishable Brahman, the Absolute, the Eternal, cannot return to this world of sorrow, pain and misery. They know neither day nor night. They are one with Existence Absolute.The manifested and the unmanifested dwell in Brahman. Brahman is beyond the manifested and the unmanifested. When the world and the body are destroyed Brahman is not destroyed. The waves come out and subside, but the ocean remains unaffected. So also the worlds come and subside, but Brahman the source of everything, the source of Mulaprakriti, ever remains unaffected. Just as ornaments come out of gold and then go back to gold when they are melted, so also all the worlds come out of Brahman and go back to Brahman. Gold is in no way affected by the various forms such as earning, bracelets, anklets, etc., that have been made of it. Even so Brahman is not in the least affected by the projection and destruction (dissolution) of the worlds and the bodies of beings. It remains always as It is.