above them all The golden Sun in splendor likest Heaven Allur'd his eye: Thither his course he bends Through the calm Firmament; but up or downe By center, or eccentric, hard to tell, (III. 571-75)
Here Milton fudges the question of wheter the universe is geocentric (the Ptolemaic theory) or heliocentric (the Copernican theory). In approaching the sun, is Satan moving towards the center of the universe or away from the center ("eccentric")? A few lines later, Milton refers to how the various heavenly bodies "Turn swift their various motions, or are turn'd / By his Magnetic beam, that gently warms / The Universe" (III. 582-83). The "or" indicates that Milton is unsure whether the stars and planets move themselves, or whether the magnetic force of the sun keeps them in line. (Newton wouldn't come up with his theory of gravitation for a few years yet, which is why Milton speculates that the force in question might be magnetism.)
That Milton was undecided about the astronomical theory of his day is quite understandable. But this uncertainty presents him with a problem: what does it say about his divine Muse if he finds it "hard to tell" which astronomical theory is correct? Why cannot divine inspiration elucidate such matters? Milton attempts to answer these objections in Book VIII, when Raphael speaks to Adam about the mysteries of the universe:
This to attain, whether Heav'n move or Earth, Imports not, if thou reck'n right, the rest From Man or Angel the great Architect Did wisely to conceal, and not divulge His secrets to be scann'd by them who ought Rather admire; or if they list to try Conjecture, he his Fabric of the Heav'ns Hath left to thir disputes, perhaps to move His laughter at thir quaint Opinions wide (VIII. 70-78)
So God deliberately made the universe mysterious because he likes to have a good laugh at those silly, vain people who believe they can figure out the mechanics of his creation. It's a little bit like modern fundamentalists claiming that God placed dinosaur bones in the ground in order to test our faith.