ARTS AND SCI. DIV. VOL. VII.
although one of its sides may then be illuminated by the sun, it is only the edge which is turned towards the observer. Besides these two causes of disappearance, which are of a transient nature, and render the ring invisible only for a few days at most, there is a third cause, which generally continues in operation during a longer period of time, and produces a more lasting effect. When the planet is so situated that the plane of the ring passes between the earth and the sun, the unenlightened side of the ring is then turned towards the earth, and consequently during the whole time that the planet is in this position (which frequently extends to several months) the ring will be invisible. The same theory affords an equally satisfactory account of the different phases assumed by the appendages of the planet during the period of its visibility. It is manifest that when the plane of the ring passes through the sun, and when consequently the ring ceases to be visible, the planet, if viewed from the sun, would appear in the node of the ring. When the planet revolves from this position, the sun commences to ascend above the plane of the ring, and the latter in consequence becomes visible in the form of a very elon gated ellipse, gradually opening out in breadth. The ellipse continues to approach towards a circular form until the planet has reached a distance of 90° from the node of the ring, when the elevation of the sun above the plane of the ring has attained its maximum. The ring henceforth begins to contract, and the same succession of appearances in a reverse order will obviously ensue as the planet revolves towards the opposite node, where the ring again will cease to be visible. The ring will therefore complete the cycle of its phases in a period equal to half a revolution of the planet, or in about fifteeu years. The appear ances will not be materially different whether the ring be viewed from the earth or the sun, except during the time that the planet is in the vicinity of either of the nodes of the ring. At such a juncture, the
combined motions of the earth and the planet may cause the plane of the ring to pass more than once through the earth, and the ring may in consequence disappear and reappear twice before its plane has entirely swept over the terrestrial orbit.
Huyghens was originally under the impression that the plane of the ring is parallel to the equator, and he consequently supposedits inclina tion to the ecliptic to be 23' 30'. On a subsequent occasion, however, he determined the inclination by actual measurement in conjunction with Picard and he found it to amount to 31°. Huyghens also fixed the ascending node of the ring in 170° 30' of longitude. Several suc ceeding astronomers determined the elements of the ring. The most recent as well as the most complete investigation of the subject is due to Beasel. The conclusion at which he arrived was that iu the begin ning of the year 1800 the longitude of the ascending node of the ring was 166" 53' 8'9 and that the inclination of its plane to the ecliptic was 23° 10' 44'7. He also found that the node of the ring retreats upon the plane of the ecliptic at the rate of 46'462 annually.
In 1675 Cassini found that' the ring around Saturn consists in reality of two distinct rings separated from each other by a dark interval. This interesting fact was subsequently established beyoud doubt by Sir William Herschel. The following are the dimensions of the planet and the two rings as assigned by M. Struve:— These measures are supposed to refer to the mean distance of the planet from the sun. At such a distance an object which would sub tend an angle of only 1" would measure 4387 miles. This would give 78,927 miles for the diameter of the planet. The absolute dimensions of the rings will of course bear a similar proportion to their respective angular measures. Thus, it will be found that the diameter of the exterior ring amounts to 175,928 miles.