What are the differences between epic poetry and tragedy according to Aristotle's Poetics?

The first difference that matter is that of length.
Tragedy, by its very nature, is more concentrated and compact. Hence its size is much
more limited than that of. the epic. The length of a tragedy is based on the principle
that the work must be short enough to be grasped as an artistic whole. This holds good
for the epic as well. But the length of the epic can be greater than that of the
tragedy. The time limits of epic are not fixed. The epic has another advantage : it can
relate a number of incidents happening simultaneously to different persons at the same
time. Tragedy cannot show more than one incident happening at one place at one time.
This is what gave rise to the concept of the Unity of Place. Though Aristotle does not
stipulate this Unity at any time, not even in the chapter concerning the epic and the
tragedy, later critics have attributed it to him. All that Aristotles says, is that
tragedy cannot represent more than one incident at one time, and that it cannot show
incidents happening at different places at the same time. This is a common sense
observation based on the practice of the Greek theatre. The greater size (length) of the
epic allowed it more grandeur and dignity in the treatment of its incidents. The
incidents in tragedy have necessarily to “be shorter, and more concentrated. The
introduction of the different episodes in an epic make it more interesting, as they
relieve the dullness and monotony. Tragedy can make use of a greater variety of metres,
while the epic has to content itself with the heroic metre. The heroic metre, or the
hexameter1 is most dignified and stately. It can make use of rare and strange words.’
The tragic mode allows the use of metaphors, in the iambic* and trochaic3 tetrameter4.
Nature, says Aristotle, has established the appropriate metres for all forms of poetry.
The iambic verse is close to the speech of men, and suited to imitation of men in
action. The epic allows greater scope for the marvellous and the irrational.
Tragedy,’however, cannot make too much use of the marvellous within the action, for this
would seem improbable and unconvincing. Epic .can relate improbable tales because it is
not going to be presented on stage before the eyes of the spectators. The degree of the
irrational can be greater because it is left to the imagination, and not placed before
the eyes. Indeed, the element ofv marvellous adds to the artistic pleasure and wonder of
the epic. Such incidents of the marvellous, which include the supernatural and the
irrational, have to be placed outside the action of tragedy. The epic uses the mode of
the narrative, and tragedy the mode of the dramatic. The plot of epic, as of tragedy,
must have unity. Yet within the overall unity, the epic allows for more and longer
incidents than does tragedy. The epic allows multiplicity of stories, which would be
unthinkable in the tragedy. The elements which are, however, only to be found in the
tragedy, are Music and Spectacle. Tragedy has a vividness which is absent in epic. This
is so, even if the tragedy is read and not acted out on stage.