While attempting to find out how to determine if a ray intersects a sphere, I came across the term "closest approach". The context it was used in was something like, "find the closest approach (Thc) of the ray with the sphere". Does anyone know what Closest Approach means? If so, does anyone know how it can be calculated? Thanks.

the "closest aproach" simply is the location where the ray is closest to the center of the sphere. if the distance there is below the radius, your ray intersects the sphere.

check http://www.gamedev.net/reference/articles/article1026.asp for that intersecting stuff...

Yeah, what OmniBrain said. It means that you first calculate the nearest point on the ray/line to the center of the sphere. Next, you check if the distance from this point to the center of the sphere is less than the radius of the sphere. If it is, then the ray/line and sphere intersected. There is a really good tutorial solely on this subject over at GameTutorials.com under the OpenGL tutorials section. Hope this helps!