That is true, but the sun is not an object of art.

Besides, how many times Victor Hugo denied his own doctrine by writing verses which were merely brilliant descriptions or admirable bits of imagination?
We are, however, talking of art and not of literature.

Literature becomes art in poetry but forsakes it in prose.

Even if some of the great prose writers rendered their prose artistic through the beauty and harmony of their periods and the picturesqueness of their expressions, still prose is not art in its real nature.

So, crude indecency aside, what would be immoral in prose ceases to be immoral in verse, for in poetry Art follows its own code and form transcends the subject matter.
That is why a great poet, Sully-Prudhomme, preferred prose to verse when he wanted to write philosophically, for he feared, on account of the superiority of form to substance in poetry, that his ideas would not be taken seriously.