Is there a sentence fragment?
At a latitude of seventy degrees north, Tromsø is Norway's northernmost city. Moreover, it is one of the best places on the planet to view the northern lights, or aurora borealis.

 - no
 - yes
There is not a sentence fragment. These are complete sentences because they express complete thoughts.
At a latitude of seventy degrees north, Tromsø is Norway's northernmost city. Moreover, it is one of the best places on the planet to view the northern lights, or aurora borealis.