Why might it be appropriate to equate civilization with state control?

Because civilization requires rules, basically. Rules are made by an entity higher than ourself, so first there were cities, and then those cities unite together in a state. The state is the entity who has the authority to set rules in order to mantain  order and peace in the territories under its authority. So, civilization equate state control because without state control we don't have a civilization.

b) the planets year length would be longer

hope this !