Sean O'Neil's Atmospheric Scattering Shader Woes

I have a nasty habit of trying to learn something new with the most complicated method possible. In this case I’m trying to learn shaders by implementing atmospheric scattering.

I’ve been trying a couple implementations of Sean O’Neil’s shaders in glsl, but none of them work. O’Neil’s pure implementation creates a black sphere, and this one does not show up at all. I was hoping if someone had a spare moment to check out my code and see what I am doing wrong. Here is the full thing, oversized textures and all