Punctuating this fragment is difficult. The natural reading is the one given here, Burnet's translation.
Kahn, however, was not pleased with that reading. He punctuated as follows: "The limits of dawn and evening are the Bear; and opposite the Bear [is] the guardian of bright Zeus."
Burnet's translation says that there are two limits:
the Bear and the guardian of bright Zeus.
Kahn's translation says that there is one limit: the Bear.