Of the 88 constellations, just under half are relatively modern inventions, dating from the late 16th to the mid-18th century. The rest come to us from Ancient Greece, most with a mythological backstory.
The Greeks inherited many of their constellations from more ancient cultures (including Egyptian and Babylonian). Thus, some major Greek characters are represented (e.g. Hercules and Perseus), while some are conspicuously absent (e.g. Jason and Odysseus), and some constellations have no clear mythological origins at all (e.g. Lepus, the Hare, who seems to possess no more significance than being Orion’s prey).
The Gemini Twins, Castor and Pollux, appear to be a Greek reboot of the Babylonians’ Great Twins, a dual personification of Nergal, roughly the equivalent of the Greek Hades, god of the underworld. To the Greeks, however, they were born of one mother (the mortal Leda) and two fathers (Zeus, king of the gods, and Tyndareus, king of Sparta and Leda’ spouse).
The most relevant Gemini myth to sky watchers is how they came to be in the sky. The twins were doomed to be separated by death (since Pollux had inherited Zeus’s immortality, but Castor had not), which caused Pollux such great dismay that he offered to bequeath his immortality to his brother.
Instead, Zeus made both “semi-immortal,” placing them among the gods (stars) for part of the year — winter, when they are prominent above Orion’s head — and consigning them to Hades’ realm for the remainder (when they “die” by invisibly accompanying the sun on its daily journey into the underworld in summer).
Gemini is easy to spot: Face southwest around 9:30 p.m., and look almost directly overhead. There you’ll see two stars representing the Twins’ heads: Castor on the right and (slightly-brighter) Pollux on the left.
Next column: Measuring star distances.