A Background node presents a virtual sky above and land (or sea, or space) below.
Background nodes describe groundColor and skyColor gradients on the ground hemisphere and sky sphere.
Background nodes can also apply panorama images on a panorama box.
Of interest is that the viewer can look around and navigate, but never get any closer to the background.
Backgrounds are faster to draw than if you used shapes to build them.
These X3D scenes are adapted directly from the
original VRML 2.0 Sourcebook chapter examples.
Introduction to VRML97 SIGGRAPH98 course notes.