Composers and the Voice, Libretto Analysis 1 - Tosca - part 2 (macrocosmic)

Let's look at the big structure of the libretto to "Tosca."  3 Acts.  Why 3 acts?  Why not 2?  Why not 4? Act 1 - The Church of Sant' Andrea della Valle - noon

Act 2 - Pazazzo Farnese - that evening

Act 3 - Castel Sant' Angelo - 4:00 the next morning

What do each of these acts share in common?  What strikes you as notable in the storytelling?  Go!