Complex sentence: The Petrozavodsk phenomenon was a series of sightings of unidentified flying objects on September 20,1977 over a vast territory from Copenhagen and Helsinki on the west to Kyzyl-Ozyok and Vladivostok on the east.
Split into simpler sentences: The Petrozavodsk phenomenon was a series of sightings of flying objects of disputed nature on September 20,1977. The sightings were reported over a vast territory from Copenhagen and Helsinki on the west to Kyzyl-Ozyok and Vladivostok on the east.