The terrestrial bodies move to a particular place, cause of generation does not.
The terrestrial bodies do not terminate all the potency of their matter, but the cause of generation does (it cannot become another, only be or not be.)
The terrestrial bodies are corruptible; the cause of generation, though bodily, is not.
The terrestrial bodies move to a particular term, the cause of generation does not, but is infinite.
The terrestrial bodies, if living, can account per se only for the generation of something like them in kind, and only as an individual; but the sun, a cause of generation, can account for the generation of many species, even as a species.
(Though St. Thomas judged that the sun was the cause of generation, most of the properties that he imutes to it can be verified of entities that are recognized, and essential to modern Physics.)