Briefly describe the cosmology of Paradise Lost

The cosmology of Paradise lost is based on the Ptolemaic system of the Universe. This means that Milton believed the Earth to be the center round which revolved around the spheres of the Seven Planets - the moon,  Mercury,  Venus,  the Sun,  Mars,  Jupiter, and Saturn. The eighth sphere was the firmament of the fixed stars. The ninth or crystalline sphere was enclosed in the tenth,  the Premium mobile, or the last hollow shell. They all encircled the Earth with a complex combination of their separate motions invented to explain the phenomenon of the heavens.

According to Milton's cosmology, the whole created universe is suspended from Heaven by a golden chain. Various insulating layers are provided between the spheres. Among these are the chaos and the Old Night,  through which Satan has to fly to reach Paradise. It was believed that the four elements which made up all earthly matter (earth, air, fire, and water) were naturally repugnant to each other and that only the imposition of God's will made them bend to natural laws,  such as that whereby the waters of the Earth do not rise up and swamp all land. Chaos is the land where these laws do not apply; so   Satan has to battle with a vision of total anarchy in flying through the chaos. This system of Ptolemy appears ludicrous to us nowadays,  but because this system was invented as a logical response to how man saw his place in the cosmos,  it makes a perfect setting for Paradise Lost.