Page:Popular Science Monthly Volume 51.djvu/373

Rh distance just under a diameter and a quarter from the planet's center. Within this distance, then, no satellite of any considerable size can circulate for the reasons above stated.

Now, the most remarkable fact remaining is that the outer edge of Saturn's ring system lies just within this limit, so that the conclusion as to its nature seems to point to the "meteoric theory," as it is called, as the only possible one. Either a satellite has been drawn within the fatal circle and disrupted, or the materials now present as a ring have been prevented from uniting to form a single satellite, as they might otherwise have done.

So much, then, for theory. The next point is. What proof can we get to substantiate it? This might seem at first a hopeless task, but that wonderful instrument, the spectroscope, has recently given us direct testimony on the subject.

One of the peculiarities of the spectroscope is its ability to detect the motion of a luminous body in the line of sight, by the shifting of the dark (Fraunhofer) lines of its spectrum from their normal position as seen in the spectrum of direct sunlight. Advantage was taken of this fact by Mr. J. E. Keeler, who obtained photographs of the spectrum of Saturn and its rings which plainly showed that the shifting of the lines due to the motion of the rings was greater in each case for the inner edge than for the outer, proving conclusively that the portions of the ring nearer the planet move faster than those farther away.

Let us see what this means. In the first place, if we suppose the rings to be solid, it is evident that they must rotate as a whole, the angular velocity of all parts being the same, but the linear or actual velocity being much greater at the outer edge of the ring than the inner, because of the greater circumference of the circle traveled over in rotation.

If, on the other hand, the ring is composed of separate particles, each in effect a little moon, it is apparent that the nearer these tiny satellites are to the planet the faster they must revolve to overcome the increasing pull of the planet and save themselves from being drawn to destruction upon its surface. In this case, therefore, the inner edge of the ring will have a much greater velocity than the outer.

Thus we see that the two theories require opposite conditions to obtain, and that the proof given by the spectroscope confirms directly the approximate correctness of the "meteoric theory."

This latter theory offers a ready explanation for the curious "crêpe" ring. Shading off gradually as this ring does from the inner edge of the bright one, it is natural to suppose that it is a portion of the former ring in which the fragments or "meteorites" are more sparsely distributed, their numbers growing gradually