. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. sition, the rings will be projectedagainst the sky as an arch with the enormous angular breadthof about 15°, which is about 30 times the diameter which thesun presents to us. 220. Disappearance of the Bings.—As Saturn revolves aboutthe sun, the plane of its rings remains, like the plane of theearths equator, fixed in space, and intersects the plane of theecliptic in a line which is called the line of nodes of the Fig. 71, let S be the sun, ABGD the orbit of the earth, andEHLN the orbit of Satur
. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. sition, the rings will be projectedagainst the sky as an arch with the enormous angular breadthof about 15°, which is about 30 times the diameter which thesun presents to us. 220. Disappearance of the Bings.—As Saturn revolves aboutthe sun, the plane of its rings remains, like the plane of theearths equator, fixed in space, and intersects the plane of theecliptic in a line which is called the line of nodes of the Fig. 71, let S be the sun, ABGD the orbit of the earth, andEHLN the orbit of Saturn. Let HN be the line of nodes ofthe rings, and draw the lines GO and KM parallel to HN, andtangent to the earths orbit. When the planet is at H, the plana RINGS OF SATURN. 179 of the rings passes through the sun, and only the edge of therings is illuminated. In such a case the rings cannot be seen,or at all events can only be seen, in very powerful telescopes, asan exceedingly narrow line. Furthermore, if, while the planetis within the lines GO and KM, the earth encounters the plana. ?ig. n. of the rings, they will not be visible. And thirdly, if, whilethe planet is within the same limits, the plane of the ringspasses between the earth and the sun, the dark side of therings will be turned towards the earth, and they will not beseen. When the planet is beyond these limits, it is evidentthat the rings will always be visible, and will present an ellip-tical appearance, as represented at E. Now, we can readily compute the length of time which Saturnrequires in passing through the arc GK. For in the triangleCSK, right-angled at 0, we know the sides CS and KS, or thedistance of the earth from the sun and that cf the planet, and cantherefore obtain the angle CKS. It will be found to oe about 6° angle is equal to the angle KSH, and therefore doublethis angle, or 12° 2, is the angle through which Saturn movesabout the sun in passing through the arc GK. Now we knowthat Satur
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