The main idea is to draw in selected 3D planes and then project onto the canvas coordinate system with an appriopriate transformation. Some highlights:

- parametrization of drawings
- usage of pgf math engine for calculation of projection transformations and

transitions points from visible (solid lines) to invisible (dashed lines) on

meridians and latitude circles - definition of 3D plane transformation with expanded styles so that they are robust

against redefinition of macros used in their construction - usage of named coordinates (nodes) for definition of characteristic points in

local coordinate systems so that they are accessible outside of their plane of

definition - calculation of intersections points with TikZ intersection coordinate system
- usage of 'to' path operation instead of 'arc' for marking angles to allow for

easy positioning of text labels on the curve - 3D lighting effects with shading

Enjoy!

Cheers,

Tomek

EDIT: uploaded an updated version that fixes some incompatibilities with the upcoming PGF version

EDIT 2: By courtesy of Kjell those examples are now also a part of his gallery.