476 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



distances of some of the more remote of these 

 bodies from the sun exceed the greatest distances 

 of some of the nearer. Thus Harmonia, at her 

 greatest distance from the sun, is about 217,- 

 000,000 miles from him; Nemansa, 231,000,000; 

 Feronia, 233,000,000, and so on; while Cybele, 

 at her nearest, is 276,000,000 miles from the 

 sun; Doris, 262,000,000; Hygeia, 259,000,000, 

 and so on. So that Cybele, at her nearest to the 

 sun, is farther from him by nearly 80,000,000 

 miles than Harmonia at her nearest. The two 

 orbits do not even approach each other within 

 this distance, enormous though it is, for the place 

 of Cybele's nearest approach is not nearly in the 

 same direction from the sun as the place of Har- 

 monia's greatest recession. The two orbits no- 

 where approach within a distance less than that 

 which separates our earth from the sun. If the 

 two planets were originally parts of a single one, 

 their orbits after the explosion would have inter- 

 sected. It is utterly impossible that, if this had 

 been so, subsequent perturbations could have 

 separated the paths by so enormous a distance 

 as 90,000,000 miles at the place even of nearest 

 approach. 



But while the discovery of multitudinous 

 members of this ring of worlds has rendered 01- 

 bers's theory of the explosion of a single planet 

 between Mars and Jupiter utterly untenable, it 

 has brought to our knowledge a remarkable rela- 

 tion which points very clearly to the real origin 

 of the ring system of planets. 



When as yet only half as many asteroids had 

 been discovered as are now known, Prof. Kirk- 

 wood, of Bloomington, Indiana, arranging these 

 bodies in the order of their mean distances from 

 the sun, noticed that certain gaps exist, in such 

 sort that no asteroids travel at or nearly at cer- 

 tain mean distances from the sun. And looking 

 more closely into these missing distances, he ob- 

 served that they correspond to the distance of 

 the giant planet Jupiter in this way, that a planet 

 traveling at any one of these missing distances 

 would have motions synchronizing with those of 

 Jupiter, in the same sense in which the vibrations 

 of one note synchronize with the vibrations of 

 another in harmony with it. For instance, there 

 is a well-marked gap at a distance from the sun 

 exceeding our earth's in the proportion of 5 to 

 2 ; now a planet traveling at this distance would 

 make three circuits while Jupiter makes one. 

 There is another gap at a distance somewhat ex- 

 ceeding three and a quarter times the earth's ; 

 and a planet at this distance would travel twice 

 round the sun while Jupiter travels once round 



him. Still more remarkable, because occurring 

 in the very heart of the ring, is the gap corre- 

 sponding to the distance of a planet which would 

 travel five times round the sun while Jupiter 

 travels twice round him. There are two gaps, 

 also, where a planet would travel seven times 

 round — 1. During two circuits; and, 2. During 

 three circuits, of Jupiter. 



Before inquiring into the meaning of this pe- 

 culiarity, we note that now, when twice as many 

 asteroids have been discovered, the peculiarity is 

 better marked even than when Kirkwood first 

 noticed it. He was justified in saying, as he did 

 in 1S68, that the coincidences are not accidental ; 

 for the odds were enormously against the observed 

 arrangement, and its accidental occurrence so un- 

 likely as to be practically impossible. But had 

 the arrangement been accidental with the eighty- 

 seven asteroids known to Kirkwood, it could not 

 but have happened that some of the eighty-nine 

 since discovered would have had mean distances 

 corresponding to those gaps or lacunce. This, 

 however, has not only not happened, but the ag- 

 gregation of asteroids at distances where Kirk- 

 wood had already noticed that they were most 

 numerous, has become still more decided. 



"We are led back, in our inquiry into the sig- 

 nificance of this singular relation, to the time 

 when our solar system was gradually forming 

 from its former nebulous condition. Imagine a 

 ring of nebulous fragments, not as yet gathered 

 into a single mass. The process of aggregation 

 would depend in considerable degree on the dis- 

 turbances to which the fragments were exposed. 

 If they were all moving in concentric orbits, and 

 were not disturbed at all, there would be no col- 

 lisions, and they would remain as a ring of frag- 

 ments. It might seem, then, at a first view, that 

 the zone of asteroids was most favorably placed 

 for aggregation into planet form, being under the 

 special perturbing influence of Jupiter, the migh- 

 tiest of all the planets. But excessive disturbance 

 would be by no means favorable to the formation 

 of a single planet. The nebulous matter must be 

 churned by perturbations, but it must not be 

 scattered by them ; and this is what Jupiter's 

 action on the planetoidal ring has done. Quan- 

 tity of matter, again, would be a very important 

 point in the process of aggregation. A region 

 crowded with nebulous fragments would soon 

 teem with aggregations, which would before long 

 gather into a few large masses, which in turn 

 would aggregate into one. But in a region where 

 nebulous matter was very sparsely strewed, ag- 

 gregations would not readily form, however migh- 



