THE MAJOR PLANETS. 245 



eclipses are visible to us. We see them enter the shadow and leave' it, and \vr 

 can estimate the duration of each eclipse, and observe exactly its beginning sn-l 

 ending. These eclipses, as we shall show on another occasion, have been in- 

 strumental, not only to useful purposes in art, but also to great discoveries in 

 sciftnce. It is by them, among other means, that the longitude of places on 

 the surface of the earth is determined ; but by far the most important discovery 

 connected with these bodies, is that of the motion and velocity of light. How 

 this was accomplished we shall also explain on another occasion. It was 

 shown, however, by these means, that the velocity of reflected light was the 

 same as that of direct light. 



SATURN. 



Beyond the orbit of Jupiter, a space equal in extent to the distance of Jupi- 

 ter from the sun, is unoccupied by any planetary body. At a distance little 

 short of a thousand millions of miles from the sun, the SATURNIAN SYSTEM 

 revolves, in a period of twenty-nine years and a half, consisting of a globe little 

 less than Jupiter, begirt with two (and probably more) stupendous rings, and a 

 cortege of no less than seven moons. 



The diameter of SATURN is eighty thousand miles, and its bulk is, conse- 

 quently, a thousand times greater than that of the earth. 



DIURNAL ROTATION OF SATURN. 



The distance of Saturn is so great that it requires the most powerful tele- 

 scopes to render the marks on his disk visible, so as to discover his diurnal 

 motion. From purely theoretical views, Laplace conjectured that it was per- 

 formed in about ten hours. Sir William Herschel, by the aid of the large in- 

 struments constructed by him, inferred that it revolves in ten hours, sixteen 

 minutes, and nineteen seconds. Sir John Herschel estimates the time of its 

 rotation to be ten hours, twenty-nine minutes, and seventeen seconds. 



The axis on which it turns is, like that of Jupiter, at right angles to the di- 

 rection of the belts, but unlike Jupiter, Saturn inclines his axis to the plane of 

 his orbit in a manner similar to the earth and Mars. The consequence of this 

 '! arrangement is that the year of Saturn is varied by the same succession of 

 / seasons subject to the same range of temperature as those which prevail on our 

 globe. 



The alternation of light and darkness is the same as upon Jupiter. This 

 rapid return of day, after an interval of five hours night, seems to assume the 

 character of a law among the major planets, as the interval of twelve hours cer- 

 tainly does among the minor planets. 



The year of Saturn is equal in duration to 10,759 terrestrial days, or to 

 258,192 hours. But as the rotation of the planet is completed in less than ten 

 hours and a half, the number of Saturnian days in the planet's year must be 

 24,592. 



The distance of Saturn from the sun being above nine times that of the 

 earth, the sun's apparent diameter at that planet will be less than at the earth 

 in a like j roportion. If in the annexed figure E represent the appearance of 

 the sun at the earth, S will exhibit its appearance at Saturn. 



