The Story of the Heavens

by Robert S. Ball

But we can still make one or two more suppositions. What if it be really true that the ring consist of an incredibly large number of concentric rings, each animated precisely with the velocity which would be suitable to the production of a centrifugal force just adequate to neutralise the attraction? No doubt this meets many of the difficulties: it is also suggested by those observations which have shown the presence of several dark lines on the ring. Here again dynamical considerations must be invoked for the reply. Such a system of solid rings is not compatible with the laws of dynamics.

We are, therefore, compelled to make one last attempt, and still further to subdivide the ring. It may seem rather startling to abandon entirely the supposition that the ring is in any sense a continuous body, but there remains no alternative. Look at it how we will, we seem to be conducted to the conclusion that the ring is really an enormous shoal of extremely minute bodies; each of these little bodies pursues an orbit of its own around the planet, and is, in fact, merely a satellite. These bodies are so numerous and so close together that they seem to us to be continuous, and they may be very minute—perhaps not larger than the globules of water found in an ordinary cloud over the surface of the earth, which, even at a short distance, seems like a continuous body.

Until a few years ago this theory of the constitution of Saturn's rings, though unassailable from a mathematical point of view, had never been confirmed by observation. The only astronomer who maintained that he had actually seen the rings rotate was W. Herschel, who watched the motion of some luminous points on the ring in 1789, at which time the plane of the ring happened to pass through the earth. From these observations Herschel concluded that the ring rotated in ten hours and thirty-two minutes. But none of the subsequent observers, even though they may have watched Saturn with instruments very superior to that used by Herschel, were ever able to succeed in verifying his rotation of these appendages of Saturn. If the ring were composed of a vast number of small bodies, then the third law of Kepler will enable us to calculate the time which these tiny satellites would require to travel completely round the planet. It appears that any satellite situated at the outer edge of the ring would require as long a period as 13 hrs. 46 min., those about the middle would not need more than 10 hrs. 28 min., while those at the inner edge of the ring would accomplish their rotation in 7 hrs. 28 min. Even our mightiest telescopes, erected in the purest skies and employed by the most skilful astronomers, refuse to display this extremely delicate phenomenon. It would, indeed, have been a repetition on a grand scale of the curious behaviour of the inner satellite of Mars, which revolves round its primary in a shorter time than the planet itself takes to turn round on its own axis.

Fig. 66.—Prof. Keeler's Method of Measuring the Rotation of Saturn's Ring.

But what the telescope could not show, the spectroscope has lately demonstrated in a most effective and interesting manner. We have explained in the chapter on the sun how the motion of a source of light along the line of vision, towards or away from the observer, produces a slight shift in the position of the lines of the spectrum. By the measurement of the displacement of the lines the direction and amount of the motion of the source of light may be determined. We illustrated the method by showing how it had actually been used to measure the speed of rotation of the solar surface. In 1895 Professor Keeler,[26] Director of the Allegheny Observatory, succeeded in measuring the rotation of Saturn's ring in this manner. He placed the slit of his spectroscope across the ball, in the direction of the major axis of the elliptic figure which the effect of perspective gives the ring as shown by the parallel lines in Fig. 66 stretching from E to W. His photographic plate should then show three spectra close together, that of the ball of Saturn in the middle, separated by dark intervals from the narrower spectra above and below it of the two handles (or ansæ, as they are generally called) of the ring. In Fig. 67 we have represented the behaviour of any one line of the spectrum under various suppositions as to rotation or non-rotation of Saturn and the ring. At the top (1) we see how each line would look if there was no rotatory motion; the three lines produced by ring, planet, and ring are in a straight line. Of course the spectrum, which is practically a very faint copy of the solar spectrum, shows the principal dark Fraunhofer lines, so that the reader must imagine these for himself, parallel to the one we show in the figure. But Saturn and the ring are not standing still, they are rotating, the eastern part (at E) moving towards us, and the western part (W) moving away from us.[27] At E the line will therefore be shifted towards the violet end of the spectrum and at W towards the red, and as the actual linear velocity is greater the further we get away from the centre of Saturn (assuming ring and planet to rotate together), the lines would be turned as in Fig. 67 (2), but the three would remain in a straight line. If the ring consisted of two independent rings separated by Cassini's division and rotating with different velocities, the lines would be situated as in Fig. 67 (3), the lines due to the inner ring being more deflected than those due to the outer ring, owing to the greater velocity of the inner ring.