The Sun (n.) is not exempt: its value is fixed only by its surroundings, which is why some languages cannot say 'sit in the sun.'
The Sun (n.) still falls under the general law of metaphoric value, the most natural referent there is.
The Sun (n.) casts its flame with no name of its own, so the act has to be borrowed from sowing.
The Sun (n.) is the sensory thing that can always disappear, so every metaphor that carries it brings only improper knowledge.
The Sun (n.) is no longer natural: taken literally it is already a metaphor, a chandelier of sorts.
The Sun (n.) is the offspring of the good, the visible son that resembles its father.