Still, many astronomers are disposed to think that the majority of comets do not travel in elliptical but in parabolic, and a few in hyperbolic, orbits. This calls for a few words of explanation. Ellipses, parabolas, and hyperbolas are all conic-section curves, but the ellipse alone returns into itself, or forms a closed circuit. In each case the sun is situated at the focus where the perihelion, or nearest approach, of the comet occurs, but only comets travelling in elliptical orbits return again after having once been seen. A comet moving in a parabola would go back into the depths of space nearly in the direction from which it had come, and would never be seen again; and if it moved in a hyperbola it would go off toward another quarter of the celestial sphere, and likewise would never return. Now it is true that the forms calculated for the orbits of the majority of comets that have been observed appear to be parabolic (a very few seem to be hyperbolic), and if this is the fact such comets cannot be permanent members of the solar system, but must enter it from far-off regions of space, and having visited the sun must return to such regions without any tendency to come back again. In that case they may pay similar visits to other suns.