Gemini is the third Zodiac sign. It is the name for constellation which shows the Dioskouroi (Polydeukes and Kastor). According to Greek Mythology Polydeukes was made immortal whereas Kastor did not. when Kastor was dying Polydeukes wished to be with his brother forever. Zeus only granted half of the wish which is why he is only with his brother at night and is so in the sky as the constellation Gemini. This myth does however vary depending on sources because according to others Polydeukes gave up his immortality to be with his brother but they are only seen at night.