In Greek Mythology, Callisto was a nymph in close association with Artemis, goddess of the hunt.

Callisto was Artemis's favorite of her attendents. While hunting with Artemis, Callisto caught the eye of Zeus, chief of the Olympian gods. Zeus resorted to trickery to win her, knowing that she had taken a vow of chastity. He came to her, disguised as Artemis. Seeing this familiar figure, Callisto let her guard down. Zeus, seeing his chance, he seized Callisto and raped her.

Callisto became pregnant by Zeus, and was, for a time, successful at concealing this from Artemis. She was fearful of the goddess's wrath for having, in a way, broken her vow. Then, however, a day came in which all of the young women who followed Artemis disrobed and bathed together. In her naked state, Callisto could not conceal her pregnancy, and so her secret was revealed to Artemis. The huntress was enraged at this, and so banished Callisto from her following.

When the time came to deliver her baby, Callisto wandered off into the forest to do so alone. At this time, Hera, the wife of Zeus, moved to get her revenge; Hera was continually outraged at the many marital infidelities of Zeus, and punished each of his victims accordingly. She threw Callisto to the ground by her hair, where Callisto transformed into a bear. Hermes took Callisto's child away to be raised by Maia, his mother.

Arcas, as the child was named, grew up to be a fine hunter in his own right. One day, when he was out hunting, he came upon the transformed figure of his mother, Callisto. She recognized him as her son, and so called out to him, forgetting her present form. Before Arcas could throw his spear, Zeus intervened. He picked up Callisto and placed her in the sky, as Ursa Major, the "great bear." Then, he picked up Arcas and set him near his mother; He became Ursa Minor, or the "little bear."

Hera was not happy with this stellar reunion. She went to her maid, Tethys, the wife of Oceanus, and asked her to further punish Callisto. So Tethys took away water and refreshing drink from the Callisto and Arcas, never letting them dip to the horizon for a drink from the oceans


1. Some sources have Callisto as the daughter of Lycaon, and that Arcas was taken to him, instead of Maia.
2. According to some accounts, Arcas killed the bear that was once Callisto.
3. The punishment of Callisto and Arcas by Tethys is the Greek explanation of why Ursa Major and Minor are circumpolar, or are visible all year long and do not go below the horizon.
Sources:
- www.zeugmaweb.com/zeugma/english/dictionary-c.htm

Thanks to fuzzy and blue for typo correction.