When the planet has moved ninety degrees farther, we again see the ring at an angle of twenty-seven degrees; but now it is the lower or southern side which is visible. When it has moved ninety degrees farther, the edge of the ring is again turned towards the earth and sun. [Illustration: Fig. 295.] The successive phases of Saturn's ring during a complete revolution are shown in Fig. 296. It will be seen that there are two opposite points of Saturn's orbit in which the rings are turned edgewise to us, and two points half-way between the former in which the ring is seen at its maximum inclination of about twenty-seven degrees. Since the planet performs a revolution in twenty-nine years and a half, these phases occur at average intervals of about seven years and four months. [Illustration: Fig. 297.] [Illustration: Fig. 298.] 272. _Disappearance of Saturn's Ring._--It will be seen from Fig. 297 that the plane of the ring may not be turned towards the sun and the earth at exactly the same time, and also that the earth may sometimes come on one side of the plane of the ring while the sun is shining on the other. In the figure, _E_, _E'_, _E''_, and _E'''_ is the orbit of the earth. When Saturn is at _S'_, or opposite, at _F_, the plane of the ring will pass through the sun, and then only the edge of the ring will be illumined. Were Saturn at _S_, and the earth at _E'_, the plane of the ring would pass through the earth. This would also be the case were the earth at _E'''_, and Saturn at _S''_. Were Saturn at _S_ or at _S''_, and the earth farther to the left or to the right, the sun would be shining on one side of the ring while we should be looking on the other. In all these cases the ring will disappear entirely in a telescope of ordinary power. With very powerful telescopes the ring will appear, in the first two cases, as a thin line of light (Fig. 298). It will be seen that all these cases of disappearance must take place when Saturn is in the parts of his orbit intercepted between the parallel lines _AC_ and _BD_. These lines are tangent to the earth's orbit, which they enclose, and are parallel to the plane of Saturn's ring. As Saturn passes away from these two lines on either side, the rings appear more and more open. When the dark side of the ring is in view, it appears as a black line crossing the planet; and on such occasions the sunlight reflected from the outer and inner edges of the rings _A_ and _B_ enables us to see traces of the ring on each side of Saturn, at least in places where two such reflections come nearly together. Fig. 299 illustrates this reflection from the edges at the divisions of the rings. [Illustration: Fig. 299.]