Mercury, the Mortal Twin

Ptolemy gives Castor the nature of Mercury, and the tradition reads it for keen intellect, writing and travel, with a recurring note of sudden misfortune, a cleaner Ptolemaic attribution than most, since here Ptolemy names the star. The pairing with Pollux is thematically neat and probably not accidental in the ancient reading: in the myth the twins are of divided nature, Castor mortal and Pollux immortal, and the tradition duly gives Castor the mild, mercurial character and Pollux the martial one. Robson supplies the modern elaboration, but the underlying Mercury-and-Mars split of the two stars is old.

Reading Castor for a "cleaner" Mercury than most Mercury-attributed stars is possible because Ptolemy names Castor and Pollux directly and individually in the Tetrabiblos rather than leaving a modern compiler to infer a delineation from a general remark about Gemini, a distinction that matters given how much of this catalogue rests on exactly that kind of inference. The Mercury-Mars division between Castor and Pollux is one of relatively few fixed-star delineations that later compilers preserved essentially unchanged from antiquity, most of the catalogue's other personality readings being considerably more recent accretions.