Hyper,
For details on how Cesium computes surface normals, start on Page 22 here: http://books.google.com/books?id=x3ttSs_bd_4C&lpg=PP1&pg=PA19#v=onepage&q&f=false
For a demo, run Source/Examples/Chapter02/EllipsoidSurfaceNormals from this repo: https://github.com/virtualglobebook/OpenGlobe
Patrick