362

THE PLANET SATUKN.

interval the plane of the rings, if extended far enough, wouldpass through the Earth (as the figure shows) whenever thedirection of the straight line joining the centres of the Earth and Saturn might lie in it. This would be the case, for in-stance, if the Earth were at e 3 and Saturn at the same timeat c ; or if the Earth were at E t and Saturn at d ; or if theEarth were in any intermediate position at e 2 , and Saturn atthe same time at f.

It is, moreover, evident that the time in which Saturn passesthrough this critical portion of its path, from c to i>, is verynearly equal to that in which it travels through a distance equal tothe diameter of the Earth ’s orbit (he., through about 186,000,000miles), since CD is, except for the curvature of the path, equalto E!SE 3 . It may be very easily calculated that this time israther more than a year.

It is also an important fact, as a little further considerationwill show, that it is only during such an interval that the Sun and the Earth can ever he on opjiosite sides of the plane ofthe rings. At all other times, whether it he the north side orthe south side of that plane which is turned towards the Sun and is illuminated by it, that same side will be seen by theEarth. But if, for instance, the Earth were at e 3 , and Saturn at the same time at f in Fig . LXXXIY., the bright side ofthe rings, i.e., the side upon which the Sun would he shining,would be turned away from the Earth . Or, to take anotherexample, let us suppose Saturn to have passed somewhatbeyond a in the same figure, and the Earth to be approachinge. The Earth and the Sun would he both upon the same side ofthe rings, and we should see their bright side. But the Earth might soon afterwards have nearly overtaken their plane, andhe about to pass through it. The shorter diameter of therings would then become narrower and narrower, until at lastthey would be seen as an extremely fine straight line. Then,directly after the actual passage of the Earth through theirplane, their dark side would be turned towards us, and theywould become invisible. And this state of things, as we shallpresently show, may sometimes last for several consecutivemonths, until the plane of the rings again passes, either through