Part of your answer lies in understanding Book VII of The Republic and the allegory of the cave. Things are beautiful when they are closer to thier true nature or essence. They do not need to be aesthetically pleasing to be beautiful...they simply must be true. Think of two objects that have exactly the same purpose, however one of them fulfills its nature better than the other. Imagine two identically proportioned wood tables only one table has a leg that is shorter than the others....inperceptable to the eye...but if you put pressue in the table it wobbles. The more beautiful table is the one that more faithlully approaches the idea of "table" or the one that more completely fills its purpose. Read the allegory of the cave....and all of The Republic really....understanding what Plato meant by and where he was going with "forms" is crucial in your paper.

Compare dawn to dusk. Both similar in appearance, but have completely different reasons for being.

You could argue that dawn is more beautiful because it is the beginning, not the end.
While I wouldnt criticize this answer it is more poetic than accurate, if you agree with my definition of Plato's ideas of beauty. Both dusk and dawn have seperate/independant purposes....ushering in the night....ushering in the day. Comparing one dawn to another would be more accurate approach....which dawn is the closest approximation to the metaphysical "ideal dawn"....whichever dawn that is....be it dawn 1 or dawn 365....that is the most beautiful dawn.