Page:The Music of the Spheres.djvu/289

 nor solid but a multitude of very small bodies journeying close together, was made by Professor Keeler when he applied the spectroscope to determine the velocity at which the rings rotate. It was then discovered that the inner parts revolved more rapidly than the outer parts and the only way in which such a phenomenon can be explained is to accept the hypothesis that the rings are composed of separate independent bodies.

Every 15 years the plane of the ring passes through the plane of the earth and hence it is seen edgewise to the earth. It then almost disappears, being so thin as to be seen only in the larger telescopes. Seen edgewise through the 40-inch Yerkes telescope, it is scarcely more than a streak of light "like a pair of illuminated needles piercing the ball on opposite sides." This is because all of these tiny moons which fly about Saturn are moving in the same plane. We obtained this edgewise view in 1907; in 1915 the ring was opened at its widest extent while in 1921 it was again almost invisible.

The reason that this ring may be observed at so many angles is because it is inclined about 27 degrees to the plane of the planet's orbit and about 28 degrees to the plane of the ecliptic. In the course of the earth's journey and Saturn's journey around the sun, we sometimes look up at this ring and sometimes down upon it, but when the earth lies exactly in the same plane, we look only on the edge which is so thin that it becomes almost invisible. The thickness of the ring is, indeed, scarcely anything