"By center, or eccentric, hard to tell"

The dimensions of Milton's cosmology are virtually impossible to gauge -- any pictorial representation is bound to be misleading.

We really have no idea of the relative sizes of hell, chaos, and heaven. We can say that the created world - all that is "nature," including the galaxies, is tiny in the vast interminable spaces between heaven and hell. Satan sees the universe at the end of Book 2, hanging like a small jewel from heaven:

And fast by hanging in a golden Chain
This pendant world, in bigness as a Starr
Of smallest Magnitude close by the Moon.

I.e., the created universe is to Heaven as the smallest star is to the Moon.

The "distance" between heaven and hell is mostly chaos, which by definition is pretty indefinite.

Also, above the universe, heaven looms so large that it's impossible to tell its shape, as there is no point from which an eye could see enough of it to judge whether it's square, round, etc.:

Farr off th' Empyreal Heav'n, extended wide
In circuit, undetermind square or round,

Thus the rendering of these elements as in the diagram below is misleading by suggesting knowledge where none exists:

Also, as this commentator notes, it's impossible to determine what lies in the center of the finite Universe - the earth? the sun? Milton appears to leave this rather basic fact open to dispute, along with so much else:

The outer spherical shell of the universe ("whole turning", i.e., the world including the earth) is created by God out of a region of chaos (or abyss "bottomless") using a pair of golden compasses--the shell of the universe is suspended from heaven by a golden chain. A flight of retractable stairs goes down to it from heaven. Hell was a separate enclosure within chaos. According to traditional medieval ideas of cosmology (to which Milton partly subscribed), in the space between the center of the universe (which may either be the earth or the sun--Milton does not assume a geocentric universe) and the outer shell of the universe are ten concentric transparent and immaterial spheres: comprising the moon, 5 planets, sun [or earth], fixed stars, crystalline sphere, and the primum mobile or rhomb (which moved all the spheres within)...

The idea of the infinite universe as having its center everywhere and nowhere apparently has a long pedigree:

Alain de Lille, a 12th century theologian, borrowing from the Corpus Hermeticum of the 3rd Century:

"God is an intelligible sphere whose centre is everywhere and whose circumference is nowhere."

While Giordano Bruno wrote:

"We can assert with certainty that the universe is all centre, or that the centre of the universe is everywhere and its circumference is nowhere." 


(Bruno is also credited with being the first to think "the universe as a continuum where the stars we see at night are of identical nature as the sun." Wikipedia)

Pascal used the following words:

"God is a circle; His centre is everywhere, His circumference is nowhere."

Incidentally, Nicholas of Cusa worked within a similar awareness of complications deriving from careful consideration of infiinitude.

"The famous saying that God is "a sphere of which the centre is everywhere and the circumference nowhere" is, in fact, first found in a pseudo-Hermetic treatise of the twelth century, and was transferred by Cusanus to the universe, as a reflection of God, in a manner which is Hermetic in spirit..This concept was basic for Bruno, for whom the innumerable worlds are all divine centres of the unbounded universe." - Francis A. Yates, Bruno.

Cusa also anticipated Copernicus in asserting that the Earth revolved around the Sun, breaking with the Ptolemaic model:

Bartolomeu Velho's image of the Ptolemaic Universe