Well, this sucks. Apparently, the universe has already created planets that do not rotate and therefore possess "day" and "night" hemispheres. I thought I was a genius when I created such a world to be inhabited by spacecowboys who could perpetually be trapped at High Noon.
Curse you, physics!
P.S. Caveat: obviously, habitable alien planets like the newly-discovered "Gloaming" do not suck. I just wish they wouldn't rip off my ideas, you know?