To add a normal horizon go to
3D view - Modify 3D view - Sky
There are a couple of sky-textures in Sky category.
Sky texture works like normal textures, except it is stretched 360° between zenith and horizon beginning due right (East if North is up in plan) (see thread 5533). However, the sky background will only render in photos from Virtual visitor perspective (not Aerial view). The perspective may require some adjustment (see SF feature requests 668). ok
