Complex sentence: The seasonal and sidereal calendar, on the other hand, was immune to interference so Thucydides could date by the rising of Arcturus without having to wade into the confusion of disconnected city-state calendars.
Split into simpler sentences: The seasonal and sidereal calendar, on the other hand, was immune to interference. So, Thucydides can date by the rising of the star Arcturus without having to wade into the confusion of disconnected city-state calendars.