Just discovered this (and these) (though they may be known here) :

http://vimeo.com/87470811

Apart from the fascination, the technique is very interesting.

Not sure if this site fits in here, but it contains some quite fascinating "knowledge" (despite the old-fashioned style) :

http://www.skytopia.com/

^ OK - thanks skwire for the very clear, interesting and instructive reply  :Thmbsup: