There's actually a reason to that but it's buried under text in-game so I guess it's OK to answer!
The 'moon' is a huge thing that hovers across the sky really closely and keeps tabs on the people. The 'sun' functions almost the same way. They're having a marital affair on who should govern the earth. Some parts is governed by the moon while the other side is governed by the Sun. They do swap places but they don't last long. They are just doing it so plantl ife can live. Whoever wins shall absorb the other, create baby stars and wait for the next 777 years before they battle again if the world should be shrouded in semi-darkness or bright lights.
They don't follow RL physics or logic. Magic and all that...