HARD JV/CA'E' S SCIENCE- G OSSIP. 



25 



THE NEBULAR HYPOTHESIS. 



No. 



By E. P. RIDLEY. 



ET us proceed now 

 to consider the 

 cases of Uranus 

 and Neptune. No 

 trustworthy infor- 

 mation has at 

 present been ob- 

 tained as to the 

 axial inclination of 

 Mercury. The in- 

 clination of Venus 

 is also uncertain 

 and is variously 

 given from 55 to 

 75 . The inclina- 

 tion of the earth 

 is 23J , of Mars 

 27I , of Jupiter 3°, 

 of Saturn 2U . 

 The satellites of 

 Uranus travel in a plane nearly at right angles to 

 the plane of the solar equator, in which the planet 

 travels ; and if these satellites are in the plane of the 

 planet's equator, then the axis of Uranus lies very 

 nearly in the plane of the solar equator. These 

 satellites also travel in a retrograde direction, that is 

 from east to west, and hence it is assumed that 

 probably the planet revolves in the same direction. 

 If the observations on Neptune's satellites can be 

 trusted, they also revolve in a retrograde manner, and 

 so probably the planet itself rotates in the same direc- 

 tion, but its axis is not so much inclined as in the case 

 of Uranus. Mr. Spencer suggests an explanation of 

 this retrograde motion by reference to the shape 

 of the rings from which the outermost planets were 

 formed. When the solar nebula was so large as to 

 fill the orbit of Neptune, its rotation must have been 

 slower, and its figure less oblate than at later stages 

 of contraction. Now, the ring detached from a very 

 oblate spheroid which bulges greatly at the equator 

 must obviously be shaped like a flat quoit as is the 

 No. 302. — February 1890. 



case with Saturn's rings, while conversely the ring 

 detached from a spheroid which bulges comparatively 

 little at the equator will approximate to the shape of 

 a hoop. Hence the rings which gave rise to Neptune 

 and Uranus, having been detached before the solar 

 nebula had attained the maximum of oblateness, are 

 likely to have been hoop-shaped, and considering the 

 enormous circumferences occupied by these rings, 

 compared with the moderate sizes of the resulting 

 planets, they must have been'very thin^hoops. Now, 

 in such a hoop the angular velocities of the inner 

 and outer surfaces respectively will be nearly equal, 

 and the planetary mass into which such a hoop 

 concentrates will have its greatest diameter at right 

 angles, or nearly so, to the plane of its orbit, so that 

 its tendency to rotate in the line of its revolution will 

 be so slight as to be easily overcome by any one of a 

 hundred possible disturbing circumstances. Besides, 

 we certainly should expect the equator to form in the 

 direction of the greatest diameter, whatever that 

 direction might be. Without feeling required to 

 point out the precise nature of such circumstances, it 

 may readily be seen that, in the case of the outermost 

 planets, the causes which ordinarily make the rotation 

 coincide with the line of revolution were at their mini- 

 mum of efficiency. So that the retrograde rotation of 

 Uranus, though not perhaps actually implied by the 

 hooped shape of its ancestral ring, is at any rate 

 quite in accordance with it. 



There is no regular gradation in the bulk of the 

 planets ; but John Fiske points out that if we 

 consider the thickness of the genetic rings as 

 determined by comparing the size of a planet witli 

 the size of its orbit, then from Neptune to Jupiter 

 there was a regular increase in the thickness _ of 

 the rings such as the theory would lead us to 

 anticipate. The enormous mass of Jupiter seems to 

 have exhausted the central mass, and the next ring 

 which formed into the asteroids was again hoop- 

 shaped and very thin, the thinnest of all ; but from 

 the asteroids to Mercury there is again a regular 



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