CASTOR was the twin brother of Pollux, sons of Jupiter and Leda and brothers of Helen. Together they form the constellation Gemini, the Twins, and are the third sign of the zodiac. Castor is also the name of the brightest star in the constellation, which lies in the northern hemisphere near Auriga, the Charioteer. Gemini is the night house of Mercury (Confessio Amantis, VII.1031-1050; Tetrabiblos, I.17).

The learned eagle tells the dreamer that Castor and Pollux have been "stellified," become stars, HF II.1000-1008. [Gemini: Pollux]

The name occurs initially, HF II.1006.