You see different types of funnels. The profile of the first one resembles a hyperbolic curve, the second is a half-sphere, the third resemble a parabolic curve. Try to launch a ball inside these funnels (click on the photo). As you see, the ball performs quite a complex trajectory, swinging in different directions. Only for the first (hyperbolic shape) funnel the trajectory is closed. This funnel (to a rough approximation) models the gravitational field, changing with distant r like 1/r - the one that makes Earth orbiting around Sun. This is an important observation: only a 1/r field (and also r2, like in a harmonic oscillator - a spring) assures closed orbits. Imagine what would happen with seasons on Earth if the gravitational field would be different: summer coming every year in different month! It was known from mid of XIX century, that Mercury planet performs an open orbit. Einstein in 1916 explained why: the field is slighly different from 1/r, due to a huge mass of Sun (which changes the geometry of Space). This was one of the proofs for General Theory of Relativity. We can simulate different fields by this programme (courtesy of dott. Mauro Brunelli).
You see different types of funnels. The profile of the first one resembles a hyperbolic curve, the second is a half-sphere, the third resemble a parabolic curve. Try to launch a ball inside these funnels (click on the photo).