Compute direction vector dir from camera to earth as
dir = earth.pos - camera.pos
A line that extends both ways to infinity through camera and earth can be described as
L = camera.pos + π * dir
with:
π < 0 : positions behind the camera (when looking at earth)
π = 0 : position at camera
0 < π < 1 : positions between camera and earth
π = 1 : position at earth
π > 1 : positions beyond earth (when looking at earth)