The two epics written by homer the illiad and the odyssey are considered to be what's known as primary epics. the third book written by virgil, the aeneid , is considered to be a secondary epic. why is this?

Basically this is the difference:

For Primary Epic

No single author (each is a product of the oral tradition)Written down after centuries of oral transmissionSecondary Epic A single, gifted poet such as Virgil or Milton composes a work that imitates a folk epic. The Æneid and Paradise Lost, for example, involved considerable research and have the style of earlier epics (particularly in setting, dignified speeches, and extended similes.

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