I always saw Star Wars as
Act I: Tatooine (until the point they enter hyperspeed)
Act II: The Rescue of Leia
Act III: The Destruction of the Deathstar (from the arrival at Yavin IV)
Intriguing. This breakdown is certainly completely consistent with the changes in scenery.
It also fits one interpretation of the three-act structure whereby each act is like a self-contained small story. Story 1 is Luke deciding to become a Jedi. Story 2 is rescuing the Princess. Story 3 is the Death Star attack.