According to Ovid, it was Jupiter who took the form of Diana so that he might evade his wife Juno's detection, forcing himself upon Callisto while she was separated from Diana and the other nymphs. Callisto's subsequent pregnancy was discovered several months later while she was bathing with Diana and her fellow nymphs. Diana became enraged when she saw that Callisto was pregnant and expelled her from the group. Callisto later gave birth to Arcas. Juno then took the opportunity to avenge her wounded pride and transformed the nymph into a bear. Sixteen years later Callisto, still a bear, encountered her son Arcas hunting in the forest. Just as Arcas was about to kill his own mother with his javelin, Jupiter averted the tragedy by placing mother and son amongst the stars as Ursa Major and Minor, respectively. Juno, enraged that her attempt at revenge had been frustrated, appealed to Tethys that the two might never meet her waters, thus providing a poetic explanation for their circumpolar positions in ancient times.