The Srimad Bhagavatam is eternal and the knowledge presented within goes back to the time of the cosmic manifestation itself.........hence when the universe first was manifested from the body Of the Lord Vishnu it was 2 billion miles and after expansion has settled into its current size.
Modern science informs us the universe is expanding.
At the perimeter of each egg shaped universe there are 8 consecutive coverings of earth, water, fire, air, ether, ego, mahatattva and pradhana and these must be used in calculating sizes and dimensions.
When the Vedas where discovered by the British and the Germans their was an attempt to destroy them due to their envious nature and we only have 6% of the original Vedic literature.
The original astronomical tables are partly lost and any calculation performed in recent times by the Indians using incomplete Surya Siddhanta may be questionable.
*The current size of the universe has been given below and of course it is *much larger but as I have said any information given using incomplete Vedic tables must be approached with caution.........one must take into account who is using the tables and how they are using them.
The perfect and *complete Surya Siddhanta when used by qualified persons will give us all .... the necessary information we require.
The Srimad Bhagavatam (proper) does not give detailed astronomical data but only generalizations.....and for astronomical information one has to consult cosmological literature.
Persons who are envious of the Supreme Personality of Godhead will endeavour to discredit the Veda but alas! they shall remain outside the realm of spiritual understanding due to their envious position.
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In his Anubhäshya commentary on this verse of Chaitanya-caritämrita, Shrila Bhaktisiddhänta Sarasvati quotes from Sürya-siddhänta 12.90, "The circumference of the sphere of the Brahmändee in which the sun's rays spread is 18,712,080,864,000,000 yojanas" (SS, p. 87). Then he quotes Siddhänta-shiromani, Golädhyäya Bhuvana-kosha: "Some astronomers have asserted the circumference of the circle of heaven to be 18,712,069,200,000,000 yojanas in length. Some say that this is the length of the zone binding the two hemispheres of the Brahmända. Some Pauränikas say that this is the length of the circumference of the Lokäloka Parvata [adrishya-drishyaka-girim] (SSB1, p. 126).
Here the circumference of 18,712,069,200,000,000 yojanas corresponds to a diameter of 5,956,200,000,000,000 yojanas. This number is much larger than the 500,000,000-yojana diameter given in the Bhägavatam, and we might ask how it relates to it. According to the Bhägavatam (5.20.37),
By the supreme will of Krishna, the mountain known as Lokäloka has been installed as the outer border of the three worlds-Bhürloka, Bhuvarloka and Svarloka-to control the rays of the sun throughout the universe. All the luminaries, from the sun up to Dhruvaloka, distribute their rays throughout the three worlds, but only within the boundary formed by this mountain.
This verse reconciles the statement that the 18-quadrillion-yojana circumference is the limit of distribution of the sun's rays with the statement that it is the circumference of Lokäloka Mountain. We also note that in SB 5.20.38 the diameter of Lokäloka Mountain is stated to be half the diameter of the universe. This is consistent with the statement in Shrila Prabhupäda's purport that "according to some, this is only half the circumference." We are thus left with a picture of the universe in which the rays of the sun and other luminaries spread to a radial distance of 2,978,100,000,000,000 yojanas, and are there blocked in all directions by an enormous mountain. This mountain lies halfway between the sun and the beginning of the outer coverings of the universe. This means that the distance from the sun to the coverings of the universe is some 5,077 light-years, where a light-year is the distance traveled in one year by a beam of light moving at 186,000 miles per second and we use the Sürya-siddhänta's 5-mile yojanas.
In Chapters 3 and 4 we will say more about the possible relation between this very large universal radius and the much smaller figure given in the Bhägavatam. At present we will consider what the jyotisha shästras have to say about the radius of the universe. It turns out that the Siddhänta-shiromani, the Sürya-siddhänta, and many other jyotisha shästras give a simple rule for computing this number.
The Sürya-siddhänta gives the following rule: "Multiply the number of ... revolutions of the moon in a kalpa by the moon's orbit...: the product is equal to the orbit of heaven (or the circumference of the middle of the brahmända): to this orbit the sun's rays reach" (SS, p. 86). If we perform this calculation, we find that the circumference of the brahmända, or universe, is:57,753,336 X 1,000 X 324,000 = 18,712,080,864,000,000 yojanas
(reference source withheld)