Having accepted the fact that the planets are revolving around the sun, the next point to be inquired into is:--What are the positions of their orbits, or paths, relatively to each other? Suppose, for instance, the various planetary orbits to be represented by a set of hoops of different sizes, placed one within the other, and the sun by a small ball in the middle of the whole; in what positions will these hoops have to be arranged so as to imitate exactly the true condition of things? First of all let us suppose the entire arrangement, ball and hoops, to be on one level, so to speak. This may be easily compassed by imagining the hoops as floating, one surrounding the other, with the ball in the middle of all, upon the surface of still water. Such a set of objects would be described in astronomical parlance as being _in the same plane_. Suppose, on the other hand, that some of these floating hoops are tilted with regard to the others, so that one half of a hoop rises out of the water and the other half consequently sinks beneath the surface. This indeed is the actual case with regard to the planetary orbits. They do not by any means lie all exactly in the same plane. Each one of them is tilted, or _inclined_, a little with respect to the plane of the earth's orbit, which astronomers, for convenience, regard as the _level_ of the solar system. This tilting, or "inclination," is, in the larger planets, greatest for the orbit of Mercury, least for that of Uranus. Mercury's orbit is inclined to that of the earth at an angle of about 7°, that of Venus at a little over 3°, that of Saturn 2-1/2°; while in those of Mars, Neptune, and Jupiter the inclination is less than 2°. But greater than any of these is the inclination of the orbit of the tiny planet Eros, viz. nearly 11°.