Tei Tenga

Tei Tenga is a fictional moon where Doom was originally supposed to take place, according to Tom Hall's original Doom design document.

The moon somehow always face the same way towards it's sun, and as such it doesn't have night/day cycles. And as a result the moon is divided up into two sections. The light, and the dark side.

Although it is referenced as "the giant moon Tei Tenga", it is also later referenced as a planet.

Tei Tenga's name lives on, as it's displayed in some computer displays in Doom. There are also WADs that expand on the idea.