(The). Milton, in his Arcades, speaks of the “celestial syrens' harmony that sit upon the nine enfolded spheres.” The nine spheres are those of the Moon, of Mercury, of Venus, of the Sun, of Mars, of Jupiter, of Saturn, of the Firmament, and of the Crystalline. Above these nine heavens or spheres come the Primum Mobile, and then the Heaven of the heavens, or abode of Deity and His angels.
The earth was supposed to be in the centre of this system.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894