Bhagavad-Gita: Chapter 8, Verse 17

Rudra Vaisnava Sampradaya:

Visnuswami

Sridhara Swami's Commentary

In the Puranas the ancient Vedic histories there are statements like: The contemplative, the charitable, the dispassionate and the consequent reach a higher abode that is free from affliction above the material realms of the earthly worlds, the heavenly kingdom and the nether regions. It is known that above the three planetary systems Bhur, Bhuvah and Swah, the planetary system of Maharloka is far superior to the Earth; but if periodic destruction is anyway applicable to all material planetary systems then what is the special distinction. Anticipating such a question and in order to convey that existence on Maharloka lasts for a long duration compared to the much shorter duration of the three previously mentioned lokas or planetary systems being Bhur, Bhuvah and Swar below it. By showing that in each day of the life of Brahma which equal 4 billion, 320 million years and who lives one-hundred years according to this time scale, the material manifestation of the three worlds are created and on each night in the life of Brahma also equalling 4 billion 320 million years the three worlds are dissolved again. Both together total 8 billion 640 million.

Such is the proper method of computing time. One year of time on Earth equals one day and one night of the demigods. Computing further in gradation for them and Earth it can be tabulated that one Maha Yuga equals 4,320,000 Earth years but for the demigods that is only a durarion of 12,000 of their years. 1000 such Maha Yugas comprises Brahma's day and another 1000 Maha Yugas comprises his night. Only those who understand time in this frame of reference are actual knowers of day and night. Those beings whose knowledge is restricted to the rotations of the sun and moon are not knowers of day and night for there knowledge is limited. The word yuga in this verse is referring to a Maha Yuga refering to the four yugas of Satya, Treta, Dvarapa and Kali equalling 4 million 320 thousand years. Otherwise combining it with sahasra-yuga would have no relevance to Brahma's day and night. The Vishnu Purana states: Brahma's day consists of 1000 Maha Yugas. The name Brahma also denotes the inhabitants of his planetary system of Maharloka as well.

Brahma Vaisnava Sampradaya:

Madhvacarya

Madhvacarya's Commentary

Lord Krishna in order to establish the fact that upon attaining Him there is no further return to the material existence, He cites the potency of His unmanifest Brahman by the example of the cyclic dissolution of creation referring to Brahma's day. The literal meaning for sahasra is one 1000; but actually it means innumerable for in innumerable creations is dissolution taking place periodically. The Brahman or the spiritual substratum pervading all creation is a manifestation of the Supreme Lord and is supreme as well being the shadow of the universal form. Here Lord Krishna speaks of dissolution at the end of Brahma's day. Everything comes into creation by the unmanifest. One Maha Yugas consists of Satya Yuga, Treta Yuga, Dvarapa Yuga and Kali Yuga equalling 4,320,000 years multiplied by 1000 is 4 billion 320 million years and is a day of Brahma. The same duration is his night both combined equal 8 billion 640 million years and which is known as a kalpa.. A year of Brahma is 360 of such kalpas totalling 26 trillion 438 billion and 400 million years and Brahma lives for 100 of these years. The Kurma Purana states: For as long as the life of Brahma is a day for the Supreme Lord and after begins His night. When His night falls everything gets merged back into Him and when His day dawns everything emanates out again and creation begins anew. The Supreme Being who resides in the etheric heart of all beings.

Now begins the summation.

The word sahasra means 1000 but it also indicative of the Brahman or the spiritual substratum pervading all existence. What is the source of the Brahman? It is emanating from the Supreme Being. Only the periodic cycles of night and day are spoken of in reference to the unmanifest Brahman. When night falls after 4 billion 320 million years everything gets dissolved back into the unmanifest Brahman. Thus from the statement can be drawn a conclusion. At the end of day and night of the four-faced Brahma all the universe is not dissolved, only portions. The Supreme Lord Krishna alone is eternal. He is transcendental to the constructs of time beyond both the night and the day. The night and day of Brahma's lifetime are said to be like the closing and opening of His eyelids

Sri Vaisnava Sampradaya:

Ramanuja

Ramanuja's Commentary

Those who through the power of yoga or the individual consciousness attaining communion with the ultimate consciousness understand that Brahma's'day lasts for 1000 Maha Yugas. Each of which is one cycle of the four yugas or ages combined which are Satya Yuga lasting 1,728,000 Earth years, Treta Yuga lasting 1,296,000 years, Dvarapa Yuga lasting 864,000 years and Kali Yuga lasting 432,000 years all together totalling 4,320,000 years. When multiplied by 1000 Brahma's day equals 4 billion 320 million years. The same total applies also for Brahma's night. So combined one day and one night of Brahma equals 8 billion 640 million years and this is known as a kalpa. One year of Brahma equals 360 of these kalpas totalling 26 trillion, 438 billion and 400 million. Brahma lives for 100 of these years.

Kumara Vaisnava Sampradaya:

Nimbaditya

Kesava Kasmiri's Commentary

Lord Krishna is now giving the reason why people who reach the heavenly worlds of Brahma and others are still subjected to samsara or the perpetual cycle of birth and death because of the periodic time factors these planets are regulated by. Those who by the truth of the Vedic scriptures know that Brahma's day and night is comprised of 2000 Maha Yugas which is equivalent to the duration of the four yugas or ages being Satya Yuga consisting of 1,728,000 years, Treta Yuga consisting of 1,296,000 years, Dvarapa Yuga consisting of 864,000 years and Kali Yuga consisting of 432,000 years all totalling 4,320,000 years multiplied againg by 2000 factually know what is day and night because they know the true reality of time. All others who base their knowledge of day and night by the mere calculations of solar movements are not knowers of day and night and are limited in their consciousness. One year on Earth is equal to one day and one night in the heavenly spheres. One day and one night in the heavenly worlds multiplied by 12,000 years equals one Maha Yuga equalling 4,320, 000 years. 1000 Maha Yugas is one day of Brahma and another 1000 Maha Yuga is one night of Brahma both totalling 8 billion 640 million years and is known as a kalpa. One year of Brahma is 360 of these kalpass or 180,000 Maha Yugas which is 360 days multiplied by 8 billion 640 million which totals to 26 trillion, 438 billion and 400 million years on Earth. Brahma lives 100 of these years. One who understands this is a true knower of day and night.

So in this way all the material worlds are always temporary due to being periodically regulated by the time factor. Naturally beings who reach these worlds are subject to these periodic regulations and thus are subjected to rebirth in the worlds of mortals. This is the case in the highest material world of Brahma in Maharloka all the way down to the lower worlds such as Pataloka. It should be understood that in all material worlds there is rebirth.