Flip normals of an object (or make material2-sided)

Very simple scenario: I want to map an image of my environment (sky etc) to a large sphere, inside which the camera will reside. However the material of the sphere only shows on the outside. How can I flip it so that it shows on the inside?

Option 1: Edit the sphere mesh so that the normals point inwards. You can do that in your 3D application: if you're using Blender, go to edit mode, select all vertices and ctrl+f->flip normals. You can switch on drawing vertex normals in the mesh tools in Blender.

Option 2: Add/modify the Cull property of your material from Cull Back to either Cull Front or Cull Off.

In case anybody finds this who uses Cheetah3d, the same operation there is done by selecting the faces (not edges or vertices) you want to invert, then using Tools → Polygon → Flip Normal.

In 3dsmax u need to apply modifier Normals->Select Flip
and export/save the model.