Commentary Par I 79-81

Perhaps because humans are accustomed to seeing no measurable space-consuming object more vast than a lake or a sea, the poet compares the extended fire he saw in the sky to a watery body.  How we are to understand the exact nature of the phenomenon at which he gazed is not clear, although some believe (see C.Par.I.61-63) it is the fiery ring that surrounds the sphere of the Moon, a common fixture of medieval astronomy that would otherwise have remained unmentioned in the poem.  But there is simply no certainty in this matter.