after, when they come to model Heaven,
And calculate the stars; how they will wield
The mighty frame; how build, unbuild, contrive
To save appearances; how gird the Sphere
With Centric and Eccentric scribbled o'er
Cycle and Epicycle, Orb in Orb.--viii. 66-84.

When, with the advancement of science, astronomical observations were
made with greater accuracy, it was discovered that uniformity of motion
was not always maintained by those bodies which were believed to move in
circles round the Earth. It was observed that the Sun, when on one side
of his orbit, had an accelerated motion, as compared with the speed at
which he travelled when on the other side. The planets, also, appeared
to move with irregularity: sometimes a planet was observed to advance,
then become stationary, and afterwards affect a retrograde movement.
Those inequalities of motion could not be explained by means of the
revolution of crystalline spheres alone, but were accounted for by
imagining the existence of a small circle, or epicycle, whose centre
corresponded with a fixed point in the larger circle, or eccentric, as
it was called. This small circle revolved on its axis when carried round
with the larger one, and round it the planet also revolved, which when
situated in its outer portion would have a forward, and when in its
inner portion a retrograde, motion.

The theory of eccentrics and epicycles was sufficient for a time to
account for the inequalities of motion already described, and by this
means the Ptolemaic system was enabled to retain its ascendency for a
longer period than it otherwise would have done. But more recent
discoveries brought to light discrepancies and difficulties which were
explained away by adding epicycle to epicycle. This created a most
complicated entanglement, and hastened the downfall of a system which,
after an existence of many centuries, sank into oblivion, and is now
remembered as a belief of bygone ages.

The devices which the upholders of this system were compelled to adopt,
in order 'to save appearances,' with 'centric and eccentric,' cycle and
epicycle, 'orb in orb,' are in this manner appropriately described by
Milton, as indicating the confusion arising from a theory based upon
false hypotheses.

Continuing his reply, the Angel says:--

Already by thy reasoning this I guess,
Who art to lead thy offspring, and supposest
That bodies bright and greater should not serve